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.
Since is an interesting word. It has several uses and is used as more than one part of speech. Because of this, since has different rules about commas in different situations.
Since can be used as a preposition or adverb when talking about time. It can mean from a specific point in time until the present.
It can be used similarly to ago and mean "before the present time." It can also be used to mean "any time after a particular time in the past." When since is used in one of these ways, we do not use a comma.
- I’ve been a lot happier ever since I moved to this city.
- The theory has been long since disproven.
- We saw each other years ago, and he has since become jaded and bitter.
Since is also a subordinating conjunction. It can connect clauses to talk about changes from a particular time (e.g. She makes more money since she has a Master’s degree.) This form of since does not need a comma.
We can also use since as a subordinating conjunction to mean because. This is where the comma rule gets tricky.
We only use a comma before since if the preceding clause is negated by the new clause. A good rule of thumb is if the preceding clause contains a negative verb, use a comma.
Examples that don’t need a comma:
- I went to the store since I needed milk and eggs.
- I want a new dog since my other dog is lonely.
Examples that do need a comma:
- I didn’t get a new dress, since I had to pay rent the next day.
- He would not go to the party, since his ex was going to be there.