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If the sentence doesn’t need the clause that the word in question is connecting, use which. If it does, use that. (Pretty easy to remember, isn’t it?) Let me explain with a couple of examples.
- Our office, which has two lunchrooms, is located in Cincinnati.
- Our office that has two lunchrooms is located in Cincinnati.
The second sentence suggests that we have multiple offices, but the office with two lunchrooms is located in Cincinnati. The phrase that has two lunchrooms is known as a restrictive clause because another part of the sentence (our office) depends on it. You can’t remove that clause without changing the meaning of the sentence.
- The garage that my uncle built is falling down.
- The garage, which my uncle built, is falling down.
I can say the first sentence anywhere and the listener will know exactly which garage I'm talking about — the one my uncle built. The second sentence, however, I would have to utter, say, in my back yard, while I'm pointing to the garage that's about to fall down.
note
In non-identifying relative clauses, which usually serve to provide additional, non-essential information and are separated by commas, which is the relative pronoun that is normally used. That would be unusual.
- The last symphony (that) he composed was the ninth symphony.
- The ninth symphony, which was composed in the final year of his life, was not performed until after this death
