Troublesome English word pairs -- which and that?

When do you use which and when do you use that?


Which and that are relative pronouns we use to introduce dependent clauses.
E.g. The movies that we like to see are the nail-biting dramas.
E.g. My cousin likes Gucci shoes, which are very expensive.


In the first example above the dependent clause that we like to see describes the subset of all movies.  The dependent clause restricts the word movies and limits it to only those we like to see.
It is a restrictive dependent clause.  Because it is necessary to the meaning of the sentence so we can't leave it out, we don't set it off with commas.






In the second example the dependent clause, which are very expensive, just gives us additional information about all Gucci shoes.  No inexpensive Gucci shoes exist.  So the clause does not restrict the meaning of Gucci shoes. It is not restrictive, not necessary, and set off by commas.