A lot of people have a hard time distinguishing between the relative pronouns that and which. It’s generally agreed that which should be used to introduce nonrestrictive clauses and that should precede restrictive ones. However, there is divergence in opinions about how strict the distinction should be applied in practice.
A restrictive clause limits the possible meanings of a preceding subject in an essential way. For example, the sentence The store honored all complaints that were less than 60 days old contains a restrictive clause. A nonrestrictive clause, on the other hand, describes a noun in a nonessential way. It would be possible to remove the nonrestrictive clause and still maintain the basic meaning of the sentence. For example, the sentence Children who eat vegetables are likely to be healthy would still make sense without the word which.
Nonrestrictive clauses should be separated from the rest of a sentence by commas if they are not at the end of a sentence and by a semicolon if they are in the middle of a sentence. That’s a rule of grammar that you can learn by studying grammar rules and reading examples of good writing.
To help you distinguish between restrictive and nonrestrictive clauses, here are three exercises. For each, try to identify which sentence contains a restrictive clause and which one contains a nonrestrictive clause. Then punctuate the sentences correctly if necessary. In the exercises below, a nonrestrictive clause takes a pair of commas and a restrictive clause should always be set off by that.