What part of speech is the word 'must'? What abbreviation does it use?
It's a verb. It's a kind of weird verb, but it's a verb. (Or, if you're in the winemaking business, it's a noun; "must" is the name of the stuff you get when you squash the grapes and it's turning into wine.)
It's called a "modal". The modals are a group of auxiliary verbs that modify the meaning of the main verb. The modals are can, could, may, might, will, would, must and should. (In British English, you can include "shall") They have to be followed by a main verb in the simple or base form (the infinitive without "to") I must go to the store. He should go to the doctor She might go to the movies. They will go if you ask them.
Must can be an auxiliary verb (Man must eat to live) or an adjective (This is a must book for your reading) or a noun (This rule is a must).