Mandarin is a relatively simple language in that it does not have verb conjugations. The same form of verbs can be used for all tenses.
Méi is also used for negating Functive Verbs (action verbs) when talking about completed actions. In this situation, méi is a short form for méi yǒu and either form can be used.
Examples of Mandarin Negatives
Wǒ bú yào.
I don’t want (it).
Tā hái méi lái.
He hasn’t come yet.
Wǒ míngtiān bù qù xuéxiào.
I won’t be going to school tomorrow.