In some languages, negative words like nobody, nothing and never have to be used with a negative verb. In English, these words are themselves enough to give a negative meaning, and we do not normally use not.
I could find nothing in the cupboard. (NOT I couldn’t find nothing in the cupboard.)
Nothing matters now. (NOT Nothing doesn’t matter now.)
I could understand nothing. (NOT I couldn’t understand nothing.)
The words nobody, nothing, never etc., are rather emphatic. Instead of these words we often use phrases like not anybody, not anything, not ever etc.
I could see nothing. (Emphatic)
I couldn’t see anything. (Less emphatic)
I could see nobody. (Emphatic)
I couldn’t see anybody. (Less emphatic)
We cannot begin sentences with not anything or not anybody. Instead we use nothing and nobody.
Nobody came. (NOT Not anybody came.)
Nothing matters. (NOT Not anything matters.)
Although double negatives are regarded as non-standard, they are not uncommon in spoken English. Note that it is a common error to suppose that a double negative is equivalent to a positive.
For example, the sentence ‘I didn’t hear nothing’ doesn’t mean that ‘I heard something’. It is merely a non-standard but perfectly acceptable way of saying ‘I didn’t hear anything’.