must, can't

Must and can't can show that you have thought about the facts and are sure. Must means "I'm sure this is true". Can't means "I'm sure this isn't true". Both are followed by the base verb, most often be.

Must for obligation, must for being sure

Must also means that something is necessary. The situation tells you which meaning it has.

The opposite of must is can't

For a conclusion that something is not true, use can't, not mustn't. Mustn't means "don't do it".

With -ing

Use must be or can't be + -ing for something happening now.

Sure, possible, impossible

Must and can't are for when you are sure. When you aren't sure, use might, may or could.

About the past

For a conclusion about the past, add have + past participle.

Common mistakes

• × She mustn't be at home. The lights are off. → She can't be at home. The lights are off.
• × He must to be tired. → He must be tired.
• × It must is late. → It must be late.