mustmodal verb
Must is used to say that something is necessary or that something is almost certainly true.
necessaryused to say that something is necessary or very important to do
Must shows strong obligation. It often comes from the speaker's own sense of what is needed, while have to often points to an outside rule. Must has no past form: for the past, use had to.
verb + bare infinitive [V + inf]
- You must wear a seatbelt in the car. V + inf
- I must finish this today. V + inf
- We must be quiet in the library. V + inf
Synonyms shall
Compare have (have to also shows obligation, often from an outside rule rather than the speaker); ought; should
almost certainly trueused to say that something is almost certainly true, from what you know
Here must is a conclusion, not an order: she must be tired means you feel sure she is tired. For the opposite, that something is certainly not true, use cannot or can't: that can't be right.
verb + bare infinitive [V + inf]
- You must be tired after such a long trip. V + inf
- She isn't answering; she must be asleep. V + inf
- There's no reply, so they must have left already.
Compare can (for the opposite, that something is certainly not true, use cannot or can't)
Phrases
Usage
Must not (mustn't) and do not have to are opposite in meaning, and learners often confuse them. You must not park here means it is forbidden; you don't have to park here means it is not necessary, but you may. Must has no past or future form: use had to for the past and will have to for the future.
Common mistakes
Mustn't forbids something; don't have to means it is not necessary.
See also have (have to is a common way to express obligation); should (should gives weaker obligation or advice)