That is why comma

Contextual difference between «That is why» vs «Which is why»? [duplicate]

Please consider the below sentences.

I have flunked the exam, that is why I am attending coaching classes.
I have flunked the exam, which is why I am attending coaching classes.

Is there any change in the meaning of sentence if I replace That is why with Which is why? For me both are suitable for the above context and unable to differentiate change in the meaning.

2 Answers 2

Kris’s answer is correct, and provides a general explanation of the difference between the words «that» and «which.» I would like to add that in most contexts, «which» acts as a coordinating conjunction and requires a comma before it («This book, which is my favorite. «), while «that» can function something as a pronoun («That is why. «), an adjective («That dog. «), or a coordinating conjunction that typically does not allow a comma before it («I chose the book that was my favorite. «). Due to the non-restrictive nature of a relative clause introduced by «which,» the clause is considered a parenthetical, and therefore must be enclosed by commas, parentheses, or dashes. The lack of a comma before «that» helps indicate that the relative clause is necessary to fully specify the noun phrase, and is therefore a dependent clause tied to the preceding clause. So in most cases, «which» requires a comma, but «that» cannot have a comma before it.

In your example, however, «I have flunked the exam, that is why I am attending coaching classes» is a comma-splice. This is because «that» must connect to the noun immediately preceding it, which in this case is the exam, not the fact that the speaker flunked the exam (which is what «which» modifies, but we can only know this because it doesn’t make sense to assume that «which» modifies «exam»—syntactically, it’s impossible to tell). Since «that» does not connect to «exam,» the second clause, «that is why I am attending coaching classes,» is an independent clause (hence the suggestion in Kris’s answer that «that» should be used to start a new sentence), and therefore cannot be joined to the preceding clause with only a comma.

«that’s why» in formal essays

«I’m», «it’s» are forbidden in formal essays.

Can I use «that’s why» in the opening of my Statement of Purpose?

It sounds a little awkward to use «that is why,» though I’m not a native English speaker.

5 Answers 5

I see no problem with the phrase that’s why in informal speech, or the word that’s in an essay. However, in “Fancy flights used to fill me with euphoria, that’s why I named myself Joseph. ” you have a run-on sentence structure. To avoid it, replace that’s why with conjunction so:

Flights of fancy used to fill me with euphoria, so I named myself Joseph.

(I replaced “fancy flights” with “flights of fancy”. “Fancy flights” might refer to elaborate airplane trips or getaways, even perhaps Joseph in his coat of many colors fleeing to Egypt. “Flights of fancy” refers to imaginings, to thinking of desired things, of castles in the air.)

Formal essay encompasses a wide range of styles: everything from an article on mathematical logic to a craftily composed but deliberately chatty personal blogpost. Kris’ NGrams inform you that published usage has shifted over the past 50 years; I’m inclined to attribute this not to a substantive change but to a relative increase in the number of “semi-formal” publications.

Your best guide is to follow the practice of other essays published in the medium for which you are writing and addressed to the audience whom you are addressing. Use the language which others in that speech community use.

And if you are asking about a “formal essay” for an examination, follow the instructions of your teacher, who is your only important audience. If it’s a standardized exam, the consensus of coaches and advisors is Do not use contractions at all. The worst that can happen is that your language will come off as a little stilted, but that is not necessarily a bad thing. The purpose of these essays is to test your mastery of formal English, not colloquial English.

When is a comma appropriate when using «that is»

Is there a better way of structuring the following?

In the first instance, efficiency needs to be evaluated; the most efficient choice, that is, the choice that minimises unnecessary requirements, is the preferred outcome.

I often use the phrase ‘that is’ but I’ve become increasingly aware that I may not be using it correctly. When writing, I sometimes feel I use too many commas. Would removing the comma after ‘that is’ be more appropriate?

EDIT

In light of the answers, it would seem that the question reduces to personal choice of offsetting a parenthetical clause when using ‘that is’. My choice of using commas would not be to everybody’s taste, especially given a sentence with as many commas as this example.

2 Answers 2

I agree with Janus BJ’s comments above. You can also use the Latin abbreviation i.e.:

In the first instance, efficiency needs to be evaluated; the most efficient choice, i.e. the choice that minimises unnecessary requirements, is the preferred outcome.

Note that some style guides specify a comma after the «i.e.», on the grounds that this is in accord with the actual meaning of the Latin id est (which means «that is»). The ostensible logic for this is that because one would follow the English that is with a comma, it is also necessary to use one with i.e.

This seems incredibly pedantic to me (because it resolves no ambiguity), as well as intruding an unnecessary pause in the flow of the sentence. So I make no apologies for not using one. 🙂

The commas aren’t a problem per se: it’s just that the sentence has a lot of short sections, and it’s not immediately clear how they relate to each other. You say you often overuse commas, and I suspect it’s a result of trying to cram too much into each sentence: I used to have a similar problem, so I recognise the symptoms.

In general, I’d advise writing shorter sentences. Where you would normally use a semicolon or colon, break the sentence there instead. Then, when you come to a situation like this one, replace commas with punctuation that shows the relationship of the different parts of the sentence.

To take this particular example:-

In the first instance, efficiency needs to be evaluated. The most efficient choice (that is, the choice that minimises unnecessary requirements) is the preferred outcome.

(As mentioned in the comments, you can use parenthetical dashes instead, and that’s really a question of personal preference.)

You might even like to think about the rule of having one thought per sentence. Short sentences like the following aren’t always appropriate to the tone or expected style, but they can help marshal your thoughts.

In the first instance, efficiency needs to be evaluated. The most efficient choice is the preferred outcome. That is the choice that minimises unnecessary requirements.

Looking at this one-thought-per-sentence version shows us another way to write the idea: a way that doesn’t look like a children’s book.

In the first instance, efficiency needs to be evaluated. The preferred outcome is the most efficient choice: that is, the choice that minimises unnecessary requirements.

Here, I’ve swapped the order of your main point, which might not fit the context so well, but you can see how it works in other cases.

This is why/that is why [that’s why]

Albertovna

Senior Member

owlman5

Senior Member

(1) They are both in common use. There is often no clear logic behind a speaker’s preference for «this» or «that». Sometimes, speakers who feel «closer» somehow to the statement they are referring to will use «this» rather than «that».

(2) «That is why» probably is more common than «this is why» in everyday language. I hear «that’s why. » more often than «this is why. » in most conversations.

(3) They can both work as paraphrases for other words and phrases like «therefore» and «for this reason».

blackGhost

Member

Added to a previous thread.
Cagey, moderator

I already created a topic about the difference between «this» and «that» a little while ago but I have a question about a particular case.
Is there a difference between :

«It’s raining. This is why I won’t go out»

«It’s raining. That’s why I won’t go out»

And is it also possible to use:

Thanks for your help.

heypresto

Senior Member

«It’s raining. This is why I won’t go out» sounds very unnatural to me.

«It’s raining. That’s why I won’t go out» is natural and idiomatic.

«It’s raining and it’s why I won’t go out» is unnatural and probably grammatically incorrect. A grammarian may be able to pinpoint the problem with it.

blackGhost

Member

blackGhost

Member

«It’s raining. This is why I won’t go out» sounds very unnatural to me.

«It’s raining. That’s why I won’t go out» is natural and idiomatic.

heypresto

Senior Member

In formal writing you could change ‘that’s’ to ‘that is’, but not, in my opinion, ‘this is’.

Here the difference between ‘this is’ and ‘that is’ is not do do with formality, but idiom and naturalness.

blackGhost

Member

heypresto

Senior Member

blackGhost

Member

PaulQ

Senior Member

The use of «this» and «that» as pronouns is key here. Sometimes they are interchangeable; sometimes, not.

This is used for the object indicated, or the closest object, or the first or only object. «This is a dog»; [points] «This is a dog, that is a cat»
That is used for a more distant object, or subsequent objects, «That is his house over there.»; «This is a dog, that is a cat»

this/that is why = this/that is [the reason] why

One aspect of use depends upon what comes before the this is why/that is why construction:

A: «Which side of the road do the British drive on?»
B: «The British drive on the left side of the road, this is why their cars have steering-wheels on the right.»

Here you see that the question is answered and a new piece of information is added for the first time. The this refers back to that new piece of information

A: «Why do British cars have the steering-wheel on the right?»
B: «The British drive on the left side of the road, that is why their cars have steering-wheels on the right.»

Here you see that the question is answered but «that» is referring to what A said, which is more distant.

blackGhost

Member

The use of «this» and «that» as pronouns is key here. Sometimes they are interchangeable; sometimes, not.

This is used for the object indicated, or the closest object, or the first or only object. «This is a dog»; [points] «This is a dog, that is a cat»
That is used for a more distant object, or subsequent objects, «That is his house over there.»; «This is a dog, that is a cat»

this/that is why = this/that is [the reason] why

One aspect of use depends upon what comes before the this is why/that is why construction:

A: «Which side of the road do the British drive on?»
B: «The British drive on the left side of the road, this is why their cars have steering-wheels on the right.»

Here you see that the question is answered and a new piece of information is added for the first time. The this refers back to that new piece of information

A: «Why do British cars have the steering-wheel on the right?»
B: «The British drive on the left side of the road, that is why their cars have steering-wheels on the right.»

Here you see that the question is answered but «that» is referring to what A said, which is more distant.

EStjarn

Senior Member

PaulQ

Senior Member

The British do not drive on the left because the steering wheels are on the right, it is the other way round, because they drive on the left, the steering is on the right.

Grady412

Senior Member

I would use this is why if I were indicating something. I open my wallet and see I have no money in it. I show you the empty wallet and say, This is why I won’t be going shopping with you.

Or, we are standing beside the car you have just wrecked. I point to the dent and say, This is why you can’t have the car keys.

Otherwise, I’d use that is why or that’s why.

blackGhost

Member

The British do not drive on the left because the steering wheels are on the right, it is the other way round, because they drive on the left, the steering is on the right.

According to your answer, the choice between ‘this’ and ‘that’ depends on the new information but ‘this’ and ‘that’ refer to the information already known. This is what I do not understand.

Edit: By the way, should my last sentence have been «That is what I do not understand?»

EStjarn

Senior Member

Applying Swan’s logic (post #11) to PaulQ’s examples:

That:

A: «Why do British cars have the steering-wheel on the right?»
B: «The British drive on the left side of the road, that is why their cars have steering-wheels on the right.»

The reason ‘that’ is used here is because what follows ‘that’ is old information, a rephrasing of the original question.

This:

A: «Which side of the road do the British drive on?»
B: «The British drive on the left side of the road, this is why their cars have steering-wheels on the right.»

The reason ‘this’ is used here is because what follows ‘this’ is new information; there’s been no previous mention of steering-wheels.

blackGhost

Member

dukaine

Senior Member

The use of «this» and «that» as pronouns is key here. Sometimes they are interchangeable; sometimes, not.

This is used for the object indicated, or the closest object, or the first or only object. «This is a dog»; [points] «This is a dog, that is a cat»
That is used for a more distant object, or subsequent objects, «That is his house over there.»; «This is a dog, that is a cat»

this/that is why = this/that is [the reason] why

One aspect of use depends upon what comes before the this is why/that is why construction:

A: «Which side of the road do the British drive on?»
B: «The British drive on the left side of the road, this is why their cars have steering-wheels on the right.»

Here you see that the question is answered and a new piece of information is added for the first time. The this refers back to that new piece of information

A: «Why do British cars have the steering-wheel on the right?»
B: «The British drive on the left side of the road, that is why their cars have steering-wheels on the right.»

Here you see that the question is answered but «that» is referring to what A said, which is more distant.

First of all, all these sentences are run-ons. The commas need to be replaced with semi-colons, or the phrase needs to be split up into two separate sentences.

Based on how I use «this» and «that», I use «this» when I’m referring to something immediate, kind of like what heypresto said first. My husband will upset me while I’m shopping and I’ll say, «This is why I don’t go shopping with you.» In your example, I would point to the rain and say, «This is why I won’t go out.» Also, I would usually say this in response to someone else’s comment. So, if my husband said, «You’re not going out because you’re too lazy», I would then point to the rain and say, «No, this is why I won’t go out». The implication is that I’m saying «Your reason’s not the reason; this reason is». Also, «this» usually comes before the reason, like, «This is why I won’t go out: it’s raining, and I’m lazy.» Most often, there’s more than one reason if you use «this» beforehand.

I would use «that» if I were giving a verbal explanation.

«You’re not going out because you’re too lazy.» «No, it’s raining. That’s why I won’t go out.»

I would also use «that» in a case where the situation is not immediate. My husband and I could be having a conversation about shopping on our couch at home. He might say, «I just like to shop spontaneously, and just figure out what I want when I get there.» Then I would say, «That’s why I don’t like shopping with you; I like to have a plan.» It’s not immediate; we’re not shopping right that second.

I agree with heypresto’s explanation as far as normal use in the US. I also agree with blackGhost when he says that, as a general rule, «this» usually indicates closeness, and «that» usually indicates distance. «It’s» indicates more of a permanence, and used more in reference to objects instead of actions or occurrences.

«That durn rain. It’s the reason I won’t go out.»
«My asthma. It’s why I don’t run very often.»

These examples suggest long-term or permanent states conditions; therefore, «It’s» is used.

Hope that helps. Ultimately, you can use any of them, and we’ll understand you. If in doubt, I think «that» is probably the safest bet.

It is why. /that is why. /this is why.

ray8838

Senior Member

I don’t know how to use this expression. This is why /It is why/this is why I ask in this forum.

Which one is grammatially correct?

Copyright

Senior Member

All three of these would follow something you’ve already said. If you can give us the sentence you would use before this, it would help us answer.

Also, you should edit your post to reflect your subject line (which I believe is what you want).

ray8838

Senior Member

All three of these would follow something you’ve already said. If you can give us the sentence you would use before this, it would help us answer.

Also, you should edit your post to reflect your subject line (which I believe is what you want).

Copyright

Senior Member

ray8838

Senior Member

Copyright

Senior Member

ray8838

Senior Member

Why is ‘it is why” not possible?

I just found this link:

Copyright

Senior Member

Others may feel differently.

zaffy

Senior Member

Would these sound natural? Does ‘that’,perhaps, refer to a fact/an action in the past?

-I need to go home and this is why we can’t discuss the issue now.
-You lied to me last time and that is why I’m not going you talk to you again.

Ahmed Al Saady

Senior Member

Hi, everyone!
I hope everything’s alright.

I still don’t know the difference between the demonstrative pronouns («it», «this» and «that»),
and how to use them correctly,
which is why I haven’t posted the thread until now.

Well,
1. «I love the way English poets write, which is why I’ve read lots of English poems.»
2. «I love the way English poets write. This is why I’ve read lots of English poems.»
3. «I love the way English poets write. That is why I’ve read lots of English poems.»

May you kindly let me know the difference between the three sentences above?
And in the sentence number 2 & 3, should I place a comma after the word «write», or should I leave them as they are?