In standard written English, two negatives in the same clause are considered to cancel each other and end up “positive.” For instance, we might have the following:
Miss Hiss is not incapable of loving.
Senora Bel-Canto’s singing was not too unbearable.
These sentences imply that Miss Hiss is at least somewhat capable and that there was at least something slightly bearable about Senora Bel-Canto’s singing. The negatives in the following sentences, however, are not considered to express the writers’ intended meanings:
ERROR: Miss Hiss didn’t do nothing wrong.
ERROR: Nobody tells me nothing.
Double negatives like these, used to intensify, have a long history in English, and if you heard or read these sentences, you’d understand that Miss Hiss is blameless, and that someone is complaining about never being informed. But according to the conventions of writing we can use only a single negative in these situations:
Miss Hiss did nothing wrong.
Nobody tells me anything.
Other adverbs that carry negative force are barely, hardly, only, and scarcely. Combining them with another negative also makes a double negative:
ERROR: He didn’t hardly eat anything.
He ate hardly anything.
ERROR:I wasn’t able to eat only seven helpings.
I was able to eat only seven helpings.
ERROR: His venom pressure was so high that he couldn’t scarcely hiss.
His venom pressure was so high that he could scarcely hiss.