Should I use a comma before "since"?
When "since" is used as a preposition, it doesn't need a comma:
- I haven't seen him since breakfast.
- I've long since forgotten what the hotel was called.
- He had held a grudge ever since.
Sometimes "since" can be used as a subordinating conjunction in place of "because". If it starts a dependent clause after the main clause, it shouldn't be preceded by a comma:
- She bought more gloves since she was always losing them.
However, if the independent clause that comes before "since" contains a negative verb, you need a comma:
- He couldn't go to the convention, since he was needed at home.