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How do I use "which"? What are the ways to use "which"?

"I don't know which book do you want."
"I don't know the book which you want."

Which of these sentences is correct?

  • Welcome to ELL.SE. Both I don't know which book you want and I don't know the book which you want can be valid, but they mean different things. What is it you are trying to say? – choster Nov 4 '16 at 20:18
  • @choster The second one is in fact technically incorrect (see here), although most English speakers would not notice. – Σωκράτης Nov 4 '16 at 22:21
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    @Σωκράτης The distinction that you draw is not universally observed in speech, especially in non-American Englishes, and even Merriam-Webster remarks that 'which seems to have a fair degree of flexibility to it, and can perform very nicely in restrictive clauses, nonrestrictive clauses, and in annoying people who feel that it should never be found outside of a nonrestrictive clause.' – choster Nov 4 '16 at 22:45
  • @choster There I agree with you entirely, hence why it is only "technically" incorrect (I have been looking at this from a BrE perspective). In speech, there is significant free rein, and most native speakers will not bat an eyelid at either word being used. – Σωκράτης Nov 4 '16 at 22:51
  • I wanted to know how to use 'which' in speaking english for instance. "I am a shop keeper a customer came and asked about a book . that book nither I saw nor I listened somewhere. So what will be my reply for the customer. "I don't know the book which you want?" or I don't know the book which you want. – Meraj hussain Nov 5 '16 at 12:25
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I don't know.  Which book do you want? 

There are two sentences here, one statement and one question.  We can be certain that there are two clauses because there are two finite verbs. 
 

I don't know which book you want. 

You want one book.  There are several possible books.  I don't know which one of the several is the one that you want. 
 

I don't know the book which you want. 

You want a book.  I know which book you want.  I do not know that book. 

  • But: "Which of these sentences is correct?" – P. E. Dant Nov 4 '16 at 22:02
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In Brief

  1. "I don't know which book do you want."

  2. "I don't know the book which you want."

As written, the first sentence is certainly incorrect, and the accuracy of the second is contentious. In this answer, I explain how it breaks a rule that, although it certainly has its proponents, has never been consistently followed by writers. It is more popular in American English than in British English.

  1. The "do" here is incorrect. The sentence should read "I don't know which book you want." "Do you want" is an interrogative form inversion of "You do want". "I don't know which book you do want," is technically correct, but sounds clunky. Given that we dispense with the do in the declarative ("You want"), we also dispense with it here in the interrogative (although starting a sentence, "Do you want...?" is correct).
  2. The mistake here is much less obvious. The sentence should read "I don't know the book that you want." See below for an explanation as to why:

Restrictive and Non-Restrictive Clauses

Let us examine the difference between "that" and "which" when used as relative pronouns:

My house, which is made of brick, was repainted recently.

My house, that is made of brick, was repainted recently. [However, my other house, that is made of wood, was not repainted.]

Both of these sentences are correct.

In the first sentence, "which" is being used in a so-called non-restrictive clause. That is to say that the clause could safely be removed without affecting the meaning of the sentence. Non-restrictive relative clauses add extra information to the sentence, but are not necessary to its meaning. In this case, the relative clause merely adds the extra information that the person's house is brick (we may assume that they only have one house, or that it is obvious which house they are talking about).

In the second sentence, "that" is being used in a so-called restrictive clause. That is to say that the clause is crucial to the understanding of the sentence, and could not be removed without changing the meaning. In this case, it implies that (as I attempt to illustrate with the sentence in square brackets) the person making the statement has multiple houses, and wants to specify that it was the brick one that was repainted recently, not the other one(s).

Let us return to your sentence:

"I don't know the book that you want."

Here, it is very important to the meaning of the sentence that the relative clause is present. If we remove it ("I don't know the book."), the sentence becomes ambiguous, and loses the meaning it is intended to have (that the book in question is the one that you want).

Summary

If the relative clause can be removed without changing the intended meaning of the sentence, use "which".

If the relative clause cannot be removed without changing the intended meaning of the sentence, use "that".

*Caveat

As I note above, this "rule" is contentious, and is certainly not followed by all writers, not is it observed by very many English speakers. If you break it, no-one will notice. It is therefore not a rule that you should worry too much about observing carefully.

  • I would be very much obliged if someone could provide a good explanation as to why the "do" in the first sentence must be removed. Although to a native speaker it is clearly wrong, I can't for the life of me think how to explain why. – Σωκράτης Nov 4 '16 at 22:36
  • ODO notes that "in British English, the word which is often used interchangeably with the restrictive that: She held out the hand which was hurt." Also see Merriam-Webster on the matter. – choster Nov 4 '16 at 22:46
  • @choster Added an addendum to that effect. – Σωκράτης Nov 4 '16 at 22:56
  • Hint ref. yr. comment: "Do you want" is an interrogative form inversion of "You do want". There's nothing wrong with "I don't know which book you do want," and we dispense with the do in the declarative... – P. E. Dant Nov 4 '16 at 23:00
  • Not from me, but I think your downvote here resulted from "both of these sentences are incorrect". The which/that thing was contentious, but it's largely settled by now. Sentence 1 is problematic, but 2 passes muster (or mustard, as my butcher has it.) – P. E. Dant Nov 4 '16 at 23:08

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