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Do we use “which” or “that” when referring to the preceding main clause as a whole?

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.

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There's more information about this at Question #78. –  J.R. Dec 22 '12 at 9:35
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marked as duplicate by Robusto, MετάEd, FumbleFingers, Monica Cellio, Peter Shor Dec 23 '12 at 0:53

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2 Answers

answers.com records "restrictive" and "non-restrictive" use:
Use which when it introduces a new clause in the same sentence. Use that when it begins a new sentence. Thus we say:

You never know, which is why...

but

You never know. That is why...

And goes on to explain:

There is a subtle but important difference between the use of that and which in a sentence, and it has to do primarily with relevance. Grammarians often use the terms "restrictive" and "non-restrictive" when it comes to relative clauses. A relative clause provides additional information about the noun it describes, but it may be considered relevant or irrelevant to the overall point of the sentence. In other words, a restrictive relative clause, which often begins with that, is usually considered essential or restrictive. Relative clauses beginning with which may contain non-essential information and would be considered non-restrictive.

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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.

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A non-defining clause, which contains only additional information, has two commas around it. They are parenthetical commas whose function is to mark the entire clause as... well... parenthetical. They are not there because you are using which. They are there because of the parenthetical nature of the information. "The man, who was wearing a blue hat, ran away" does not use which but does have parenthetical commas, because the blue hat is just a bit of additional information. "The man ran away. He was also wearing a blue hat". –  Roaring Fish Dec 22 '12 at 11:25
    
Thanks--I didn't mean to imply that "which" requires a comma simply because such a rule is tied to the word itself, which is why I said that "which" requires a comma "in most cases," but the preceding couple sentences were not terribly clear (and contained an instance of "that" where I meant "which"), so I've rewritten them. Better? –  Kyle Strand Dec 22 '12 at 17:03
    
Much better - worth a +1 now. –  Roaring Fish Dec 23 '12 at 9:04
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