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When do you use that and which?

In current usage, that refers to persons or things and which is used chiefly for things. The standard rule says that one uses that only to introduce a restrictive or defining relative clause - one that identifies the person or thing being talked about. An example is "The fort that Keir built has to be taken down" and the clause "that Keir built" describes which fort has to be taken down, i.e. it is restrictive. In contrast, which is used only with nonrestrictive or nondefining clauses. This type of clause gives additional information about something that has already been identified in the context. An example is "The students have been complaining about the assigned novel, which is hard to understand." The clause "which is hard to understand" is nonrestrictive as it does not indicate the specific novel being complained about. In a sentence including a nonrestrictive clause, the sentence would still be clear even if the clause were omitted. One will find that which sounds more natural than that in such a sentence, which is a great double-check of the grammar. Some people very strictly use that only in restrictive clauses and which in nonrestrictive clauses. However, even in good prose one will find the use of which in restrictive clauses is very common and considered grammatically acceptable. An example is "I would like to find a website which will tell me all about writing a research paper."

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