A double negative is when two negatives used in a sentence cancel each other out to make a positive. This must not be misunderstood to mean that double negatives are an accepted grammatical device; they are grammatically incorrect and must not be used.
How does one avoid double negatives? It is quite simple. When using the negative form of a verb (e.g.aren’t, don’t, won’t, etc), do not follow it up with a negative determiner/quantifier (e.g. nowhere, nothing, never, etc). Consider the following sentences:
They aren’t going nowhere.
They don’t do nothing.
They won’t never stop.
All of the above employ the double negative, and are hence grammatically incorrect. Formerly, the double negative was a device for emphasis, but the effort to make English conform grammatically to formal logic has rendered it unacceptable today. Logically, the meanings of the above sentences, deduced by applying the formula negative + negative = positive, are:
They are going somewhere. (aren’t + nowhere = somewhere)
They do something. (don’t + nothing = something)
They will stop. (won’t + never = will)
The ban on multiple negatives also applies to combination with adverbs such as hardly and scarcely.It is therefore regarded as incorrect to say‘I couldn't hardly do it’or‘The vehicle has scarcely no fuel.’