English as an Additional Language: That vs Which

Using That vs Which

The key to understanding whether to use that or which is learning the difference between restrictive (essential) and nonrestrictive (nonessential) relative clauses.

A restrictive relative clause is a clause containing essential information about the noun that comes before it. If you leave out this type of clause, the meaning of the sentence is affected. It will result in either a sentence of doubtful meaning or a sentence that makes no sense. 

Restrictive relative clauses are typically introduced by  words like that, as well as, by, whose, who, or whom.

A nonrestrictive relative clause contains extra information that could be left out of the sentence without affecting the meaning or structure. Nonrestrictive relative clauses can be introduced by which, whose, who, or whom, but you should never use that to introduce them.

If you leave out this type of clause, the result is a sentence that may have less information but still makes sense.

Note that a nonrestrictive clause is preceded by a comma (setting off the extra information), whereas no comma should precede a restrictive clause (indicating that the information is essential, not extra): 

   E.g. They got into the van, which had Ohio plates. [nonrestrictive] 

   E.g. I was driving the van that had Ohio plates. [restrictive]