Answers to the age-old question—which is which? And when?
1. The New York cafés that I hang out in are exclusive and hip. Like me.
2. The New York cafés, which I hang out in, are numerous and varied. Like my friends.
Sentence 1 focuses on a limited group of things—the select few NYC spots good enough for you to spend your precious time in. Sentence 2 is about the entire range of coffee-serving establishments in that city of eight million stories.
When you’re uncertain as to whether a sentence requires which or that, say it to yourself: if your meaning makes you want to pause and put in those commas before and after (like 2), it’s which. And if not, go with that.
And that’s that. Easy.