Do you know the difference between THAT and WHICH? My friend Debbie likes to say this: If it sounds like you could use either, use “that.”
*Cooking is an activity that relaxes many people (CORRECT)
*Cooking is an activity which relaxes many people (INCORRECT)
In the above sentence, to the untrained ear it may sound like you could use either. So following Debbie’s advice, THAT would be the correct choice. And it is!
Here’s a technical explanation of when to use THAT and when to use WHICH:
Essential clauses, which CANNOT be removed from a sentence without changing its basic meaning, require a THAT:
*Cooking is an activity THAT I do all the time.
If you remove the essential clause above, you’d be left with:
*Cooking is an activity. (CHANGES BASIC MEANING OF SENTENCE)
Non-essential clauses, which CAN be removed without altering the basic meaning of the sentence, require a WHICH. (Note: these type of clauses, such as the ones I’ve written above, are set apart with commas.)
*Cooking, which I love, is relaxing.
If you remove the non-essential clause above, you’d be left with:
*Cooking is relaxing. (DOESN’T CHANGE BASIC MEANING OF SENTENCE)
Got it? I know this is tricky, so if you’re more confused than ever, see if the clause in question is set apart by commas. That should help you figure it out.