Negative sentences are formed by placing méi没 or méi yǒu没有 in front of the main verb and dropping the le 了.
Nèi gè wèntí wǒ méi huídáduì. 那个问题我没回答对。
I did not answer that question correctly.
The negative form may also use bù不 instead of méi yǒu没有, but this commonly occurs only in conditional clauses describing actions that have not been carried out. Compare the highlighted parts in the following sentences. One uses bù不 while the other uses méi没.
Negative form using bù不 implying a condition.
Wǒ bú zuòwán zuòyè jiù bú shuìjiào. 我不做完作业就不睡觉。
If I don't finish doing my homework, I will not go to bed.
Comparison with méi没 or méiyǒu没有, which does not imply a condition.
Wǒ méi zuòwán zuòyè. 我没做完作业。
I did not finish doing my homework.