which vs that

To continue the blog about the usage difference between who and that... which and that are different in usage as well. I don't think very many people are aware of the fact that there is a difference between them as most people speak and write them interchangeably. I did too until my Latin professor of 5 years finally kindly pointed out to me a few year ago! Which is used with nonessential clauses whereas that is used with essental ones; a clause is nonessential if it's not necessary to complete the meaning of itself while an essential clause is necessary. So if one were to use which, a comma is placed before it but not if that is used. Determing what is necessary and not can be a little confusing at first