Should I use a comma before "for"?

When "for" is used as a preposition, it doesn't need a comma:

• He made a cake for her.
• We're away for two weeks.
• It's not for me to complain.

Sometimes "for" 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 for she was always losing them.

However, if the independent clause that comes before "for" contains a negative verb, you need a comma:

• He didn't take his umbrella, for the rain had stopped that afternoon.