We use "must" when talking about something important that needs to be done. 

In American English (I'm not sure about British or Australian English), "must" isn't really used because it sounds old-fashioned. We prefer the verb "need"; "Need" is the same as "must", but can also be used with objects i.e. "Need a blanket?".

Examples with must:

    "I must finish my work."

    "Must you always talk so much?"

Examples with need (action and object):

    "I need to go to the bathroom"

    "Do you need a drink?"

This link also provides examples:

http://www.thefreedictionary.com/must

You'll notice that the following verb is always in its original form: must have, must agree, must be...