没有 (méi yǒu) serves two essential negative functions in Mandarin. First, it negates possession: 我没有车 (I do not have a car), the negative counterpart of 有 (to have). Second, it negates past action, where 不 cannot be used: 我没有去 (I did not go), not 我不去 which means "I am not going" (future/habitual).

The 没/不 distinction is one of the most important grammar points for learners: 不 negates present/future/habitual actions and states, while 没 (short for 没有) negates completed or past actions. In speech 没有 is often shortened to just 没 (méi), particularly in rapid conversation.