Is "must" an imperative verb?
No. "Must" is not an imperative. It's a Modal Verb.
An imperative would be, "Go!" whereas "You must go." is not a command, but an indication of an obligation.
Here's the opening sentence from the Wikipedia article:
"A modal verb (also modal, modal auxiliary verb, or modal auxiliary) is a type of verb that is used to indicate modality – that is: likelihood, ability, permission, and obligation. ... Examples include the English verbs can/could, may/might, must, will/would, and shall/should."
So, what does that mean?
1st: likelihood (possibility) -- "She may/might go."
2nd: ability -- "He can lift 500 pounds."
3rd: permission -- "You may have another piece of pie."
4th: obligation -- "We must arrive by 8:00 a.m."
The article also mentions "should". "You should come to the party" is not the same as saying, "Come to the party!"
The first sentence is indirect, indicating the wishes of the speaker. The second sentence is a direct order.
Check out also: http://www.englishpage.com/modals/must.html
and here's another web page to look at: