Children's, Intermediate and Advanced Online English Dictionary & Thesaurus

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parts of speech:
auxiliary verb, noun
Word Combinations (noun), Note
part of speech: auxiliary verb
definition 1: to be obliged to because of natural law or physical requirements.
We must eat in order to live.
definition 2: to be required to by moral, social, or legal obligation.
Children must attend school in the United States.
definition 3: used to express a command, or, in negative statements, a prohibition.
You can go out, but you must be back in an hour.You must not call this number unless it's really an emergency.
definition 4: used to express a logical probability or reasonable inference (followed by "have" plus a past participle when referring to an action or state in the past).
You must be Ellen's sister! You look just like her.The road is wet, so it must have rained recently.He must have been upset when he found out he didn't get the promotion.
definition 5: to be determined to by personal resolve or desire (often pronounced with strong emphasis).
I MUST get a copy of his new book!
definition 6: to be certain to.
We all must die someday.
part of speech: noun
definition: something important, extremely advisable, or essential; necessity.
Successfully completing all the lab work is a must if you want to pass this course in chemistry.Visiting the museums is a must while you're in Washington, D.C.
similar words:
necessary, necessity, need, requisite
Word CombinationsSubscriber feature About this feature
must or have to?
Have to and must share the meaning of necessity to do something. Have to is used more often in everyday conversation. Must is more often used in writing or in a formal situation. But for the meaning of must in a sentence such as You must be joking!, both have to and must are very common in conversation. You need to be careful, though, to choose between have to and must when your sentence uses not. In negative sentences, have to and must have different meanings, as you can see in the examples below.
  • You have to leave tomorrow.
  • You must leave tomorrow.
  • You do not have to leave tomorrow. (It is not necessary for you to leave tomorrow. You can leave tomorrow if you want, or you can stay.)
  • You must not leave tomorrow. (You do not have permission to leave tomorrow.)