Most of my editing clients get this one wrong, so don’t feel too bad if this is news to you, too.
How do you know when to use the pronoun “which” and when to use “that?” Its fairly simple. If you can drop the dependent clause in a sentence without losing the point, you should choose “which” and enclose it in commas.
Amy’s dog, which is thirteen years old, is having trouble catching squirrels. (If you take out “which is thirteen years old,” the sentence still works.)
However, if the clause is a critical piece of the sentence, use “that.”
The dog that is having trouble catching squirrels is Amy’s great Pyrenees.
There you have it… mystery solved!