That recently happened to me when I was asked to describe when should we use “that” and when should we use “which”. Here’s my rule of thumb which hopefully can lend some help.

 Restrictive Clause–That

A restrictive clause is just part of a sentence that you can’t get rid of because it specifically restricts some other part of the sentence.

• Cats that are black often signify bad omen.

The words that are black restrict the kind of cats you’re talking about. Without them, the meaning of the sentence would change. Without them, you’d be saying that all cats signify bad omen, not just black cats.

Nonrestrictive Clause–Which

A nonrestrictive clause is something that can be left off without changing the meaning of the sentence. You can think of a nonrestrictive clause as simply additional information. Here’s an example:

• Black cats, which are generally less popular, often signify bad omen.

Removing the clause “which are generally less popular” will not change the overall meaning. (Also note that the phrase is surrounded by commas. Nonrestrictive clauses are usually surrounded by, or preceded by, commas.) Here’s another example:

• There was an earthquake in China, which is bad news.

Expendable–Which

If you leave off the clause that says which is bad news, it doesn’t change the meaning of the rest of the sentence.

A quick and dirty tip is to remember that you can throw out the “whiches” and no harm will be done. You use which in nonrestrictive clauses, and if you eliminate a nonrestrictive clause, the meaning of the remaining part of the sentence will be the same as it was before.

Needed–That

On the other hand, if it would change the meaning to throw out the clause, you need a that. Do all cars use hybrid technology? No. So you would say,

• Cars that have hybrid technology get great gas mileage.

Is every leaf green? No. So you would say,

• 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 that the that clause isn’t surrounded by commas. Restrictive clauses usually aren’t set off by commas.)

Hope the above sets you on the right track. Cheers!