Comma Before But

A comma should be placed before the word but only if but is at the beginning of an independent clause.

I would go for a walk, but for the rain.

In this sentence, but is being used to mean except. Because but isn’t starting an independent clause, the comma isn’t required.

The dog is young, but well-trained.

Grammar is boring, but necessary.

I would go for a walk, but it’s raining outside.

Because it’s at the beginning of an independent clause, the comma before but is necessary.

The dog is young, but he’s well-trained.

Grammar is boring, but it’s necessary.