Question

when should i use that not which and vice versa

KXM · Accepted Answer

In modern use, the two are somewhat interchangeable. However, you should use a comma before "which." There is a good explanation of the traditional rules here (http://www.kentlaw.edu/academics/lrw/grinker/LwtaThat_Versus_Which.htm) It is nice to know the proper rules so that if you break them, you do so knowingly.

Fivejedjon · Answer

Ke)vin MiddletonIt is nice to know the proper rules so that if you break them, you do so knowingly.

Those 'rules' are useful guidelines, but they are not 'proper'; it is possible to produce perfectly acceptable English sentences without slavishly following those rules. More solid guidelines, which most speakers of standard forms of British English seem to accept are:

1. In non-defining/non-retrictive relative clauses, which must be separated from the rest of the sentence (in writing) by some punctuation mark, we use who for people and which for non-human things. We cannot use that:

The Prime Minister, who had an affair with his secretary, was forced to resign. (We know which Prime Minister we are talking about, and are adding more information about him.)
The Battle of Waterloo, which was fought in 1815, sealed the fate of Napoleon.

Non-defining/non-retrictive relative clauses give us more information about someone/thing in the sentence. They are not an essential part of the sentence. The punctuation marks around them, usually commas, act effectively as brackets.

2. In defining/restrictive relative clauses, which must not be separated from the rest of the sentence by any punctuation marks, we can use that instead of who or which. Some people prefer to use only who (not that) for people:

The prime minister who/that had an affair with his secretary was forced to resign. (The relative clause tells us which prime minister was forced to resign).
The battle which/that sealed Napoleon's fate was fought in 1815 at Waterloo.

Defining/restrictive relative clauses tell us precisely which person/thing is being discussed, They are an essential part of the sentence.

3. The relative pronoun can never be dropped from a non-defining/non-retrictive relative clause; it can, but does not have to be, be dropped from a defining/restrictive relative clauses when it is the object of the verb or of a preposition:

My English teacher, who(m*) I liked very much, was actually Italian. - Must not be dropped.

The English teacher [who(m*)/that)] I liked most was actually Italian. - Can be dropped.

The car [which/that] I was most fond of was my old Morris 8. - Can be dropped.

* Except in formal writing and very formal speech, objective whom is rarely used in modern BrE.

4. When which refers back to a whole clause, it cannot be replaced by that:

Simon ran away to sea, which broke the old man's heart.

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