A double negative uses two negative elements. The meaning depends on how the negatives work together. In I didn’t see nobody, for example, speakers of some English varieties use both negatives to express one negation.
Negative concord
Negative concord is a regular pattern in several varieties of English. In those varieties, I didn’t see nobody means the speaker saw no one. For standard edited American English, write I didn’t see anybody or I saw nobody to express that meaning. See negative forms for more examples.
When two negatives do different jobs
We can’t not respond means we have to respond. The first negative rules out the option of failing to respond. In The delay was not unexpected, not negates the adjective unexpected. The phrasing suggests that the delay was at least somewhat anticipated. See negation for more on what a negative word applies to.
