Which/That

This one is super easy, guys. This is one of those ones that people do wrong all the time because they think it makes them sound smarter. The joke is on them. It makes them sound dumb because it's wrong. 

If you're describing something, the word "which" will always follow a comma. 

Let's demonstrate.

The hawk is trying to catch the nuthatch which lives in that tree.

That/Who

A person is a who, not a that or a which. This is a big one for me. I'm not sure why, but it drives me nuts, and nobody seems to get it right. Let's demonstrate.

My sister is the person that gave me a cookie.

By using "that," I have effectively dehumanized my sister. While in theory that's entertaining, it doesn't make for a very good sentence. The correct sentence would be:

My sister is the person who gave me a cookie.