Must is a modal verb (see Modal Verbs), which is a form of auxiliary verb (see Auxiliary Verbs).
It is used before other verbs to say that something is necessary (ex. You must pay taxes.).
It can also be used to say that something is a very good idea (ex. You must try the new restaurant in town).
Like other modal verbs, must follows the formation pattern of other auxiliary verbs.
Subject + must + verb
Ex. We must hurry.
Must + subject + verb
Ex. Must you leave?
When using modals like must, to is not used before the infinitives (dictionary verb forms) like with other kinds of verbs. ex. I must go home. (NOT, I must to go home.)
It also does not use ~s before it when the subject is 3rd person singular. ex. He must call them. (NOT, He musts call them.).
Questions and negatives do not use do with modals. ex. Must I bring money? (NOT, Do I must bring money?)
There is no past or infinitive form of must. Instead, have to is used. ex. I had to leave early. (NOT I musted to leave early.) ex. It’s difficult to have to wake up early. (NOT, It’s difficult to must wake up early.)
See also Have To, Must Not, Need Not, Modal Verbs, and Auxiliary Verbs
the fun they had