One of those hard and fast rules we were all taught early on was that double negatives were a no-no. In most cases that rule still holds, primarily in our business writing. But in casual writing to friends and acquaintances, a sentence like “I’d love to go to the concert with you, but I ain’t got no money” will not heap scorn upon you. It will be understood that you are acknowledging yet making light of your financial situation.
Sometimes double negatives are a way to avoid hurting someone: “I didn’t say I don’t like your outfit” is a much softer way to say you are not crazy about the other person’s new clothes. It implies you see room for improvement, but you are being kind.
Remember that barely, hardly, scarcely, never, neither and nor are all negatives, so if your verb is negative (such as was not, were not, has not, have not, am not, is not) adding one of these adverbs is going to result in a sentence you probably don’t want the world to see.