Do you know when to use that and which?

It's easy but they're often misused.

Use THAT when the following clause is necessary to the sentence's meaning. Use WHICH when you can leave it off without affecting the meaning. That may sound too easy. You need to know the definition of a restrictive clause to understand it. A restrictive clause is necessary to the meaning of the sentence. It specifically restricts some other part of the sentence. Here's an example:

Some apples that taste bitter cause stomach aches. (Do not use commas.)

The words - that taste bitter- restrict the kind of apples. Without the clause, the meaning of the sentence would change. You'd be saying that all apples cause stomach aches not just the apples that taste bitter.

A nonrestrictive clause - with which - can be left off without altering the sentence's meaning. It just provides additional information.

Apples, which come in a variety of red and green, can cause stomach aches.

If we leave out the variety of red and green, the meaning doesn't change. Notice nonrestrictive clauses are surrounded by, or preceded by commas.

More examples:

We had a forecast for rain today, which is bad news. If you omit the clause, the meaning of the sentence doesn't change.

You can toss out whiches without losing the meaning.

Cars that have hybrid technology get great gas mileage.

Leaves that are green contain chlorophyll.

It would change the meaning to throw out the clause in those examples, so you need a that. (Also note the that clause does not require commas. Restrictive clauses usually aren't set off by commas.)

Notice thats and whiches today! Sweep away the whiches.