The Comma ,

To know when to use a comma, you must know a couple of things. First, the comma tells you when you should pause if you're reading out loud. If you can figure out where a natural pause would come in reading, then you know most of the time where the comma should go. If you aren't able to figure out where the natural pause comes, the following information may help you learn to use a comma in the right place:

1. If a sentence has two ideas but only the second idea can stand alone as a sentence, you need to separate the two ideas with a comma
   
   When I saw the cake, I was surprised.
   
   If you wrote the sentence with the partial idea at the end of the sentence, you wouldn't need a comma. I was surprised when I saw the cake.

2. You also use a comma if you want to add more words to describe the subject. If you don't use the comma, the meaning of your sentence changes.
   
   The woman, who was our teacher, was very nice.
   
   This words surrounded by commas tell us that the teacher was a nice woman. They are not necessary to understand the sentence, they just give the reader more information. Look at the same sentence without commas.
   
   The woman who was our teacher was nice.
   
   This sentence seems to say that there is more than one woman and that only the teacher is nice.

3. You also use a comma to separate things in a list.
   
   I went to the store to buy milk, eggs, and butter.

4. You need a comma if you're writing to a specific person.
   
   Justin, how was your trip to Vancouver?