A double
negative sentence pattern must have a negation that denies an event, state, or act
expressed by a sentence predicate and thus covers the negation scope of a sentence.

So the phrase 'I'm not bad', isn't a double negative

God's Love, this is all getting a bit advanced and mind-boggling for us average FT' I thought a double negative was when you'd dated two Chinese girls in a row and they'd both asked you for money for doing precious little that you wouldn't get down the pink rooms for half the price. Our job is only to chat to Chinese students. If they ask you what a double negative is, tell them to stop causing trouble and google it.