Here's a mistake that I see a lot -- folks using "which" when they should be using "that."
So how do you know which one to use?
Ask yourself; is the information that follows "that" or "which" essential to the sentence? In other words, is the information vital to understanding the sentence? If it is, use "that." If it's not, use "which."
In most cases, you need to use a comma before "which" and a comma after the phrase you are adding.
The crab dip that Mary made for the Christmas party was delicious.
I used "that" because the information that follows "that" is essential to understanding the sentence. I'm not just talking about any crab dip; I'm talking about the crab dip that Mary made. And believe me when I tell you that no one makes crab dip like Mary.
The Christmas ornaments, which I bought at a yard sale, look beautiful on the tree.
In this case I used "which" because the information is not essential to understanding the sentence. The phrase could easily be removed and the sentence would still make sense. I could take that information and make another sentence.
The Christmas ornaments look beautiful on the tree. I bought them at a yard sale.
If info is essential, use "that" and no commas.
If info is not essential, use "which" and commas.