Meaning is everything, and that little comma can really make a big difference in your meaning. I especially pay attention to phrases that have because and since in them. See if you can tell the difference in meaning with each pair of sentences:
- I didn’t go to the store because you were angry.
- I didn’t go to the store, because you were angry.
In the first instance, my going to the store had nothing to do with your anger, and that’s what I’m telling you. But with the comma, the sentence means I didn’t go to the store for the reason that you were angry. Your anger kept me from going to the store, which is the opposite of the first sentence’s meaning.
- I played football since I was nine.
- I played football, since I was nine.
The word since has some different meanings, so by using the comma, you’re being clear. In the first sentence, I started playing football at age nine. The second sentence gives the reason I played football—because I was nine, implying you were at a qualifiable age. So, to quote my eighth-grade English teacher, Mr. Holtby: “Say what you mean. Don’t say what you don’t mean.”