The word unless is a tricky one, but sounds so nice writers like to use it anyway. I was writing something the other day with unless, and had to stop. I stared at the screen. And I realized that I didn't know if you're always supposed to use a comma with unless or not. I had to find out. Now, I'm going to tell you.
Unless You Want to be Wrong
Unless is not strictly a conjunction, but it behaves like one. The word is used to link two separate thoughts together, two clauses, in a sentence. For example:
I'll go with you unless Johnny is going to be there.
I'll go with you and Johnny is going to be there are two separate things that are pulled into the same sentence with that important word unless.
But let's examine the sentence even more deeply. The main idea of the sentence is I'll go. That's the main thing we're talking about. But I'm adding this last part about Johnny, because his presence will affect whether or not I'll go. That makes the second part of the sentence the subordinate clause. It's subordinate because it can't exist without the main clause of the sentence -- which is I'll go.
If you understand that, you can figure out when and how to use a comma with unless, because there is a rule. When the subordinate clause comes first, you must always use a comma. When it comes second, leave the comma out. Here's the same example, but in a different way:
Unless Johnny is going to be there, I'll go.
See the difference? This time, the subordinate clause comes first. That's why a comma here is a necessity.
Unless you want to get it wrong, use a comma when the unless comes first. When you want to do it right, skip the comma unless you just don't care.