[susan53]

Double negation

I promised him that I'd go to his party so I can't not go.

This is fine. When people say that English doesn't use double negation, it's not this type of sentence with two negative verbs that they're referring to, but sentences with negative adverbs like never. In the following examples, a and b are correct but c isn't :

I don't ever go swimming. (Correct - the verb is negated so the adverb isn't)
I never go swimming. (Correct -the adverb is negated so the verb isn't)
*I don't never go swimming. (Incorrect because both the verb and the adverb are negated - a double negative construction)