Deduction: must / can't

When you make a guess based on evidence, use must if you're sure something is true and can't if you're sure it isn't true. In between are might / may / could (possible). They are followed by the base verb.

• must / can't + be + adjective, noun, place: He must be at work.
• must / can't + base verb (often know, have, live, like…): She must know him.
• must / can't + be + -ing for something in progress: They must be watching the match.

💡Only use must when the evidence is strong and you feel almost certain. For a weaker guess, use might, may or probably.

Use must when you work something out from what you see, hear or know. The evidence is often in the same sentence or the next one.

• You've been on your feet all day. You must be exhausted.

• The lights are off. They must be out.

• Minh must know the answer — he's a maths teacher.

• Listen to that noise. The neighbours must be having a party.

• You must be Lan's brother. You look just like her!

When you're guessing, the negative of must is can't (or couldn't), not mustn't — mustn't means “not allowed”. In American English people also say must not (not contracted) to mean “I conclude it isn't true”: He must not be home — but can't works everywhere and is the safe choice.

• That can't be Tuan. He's in Singapore this week.

• Ten dollars for a new laptop? That can't be right.

• She can't be hungry — she's just had a big lunch.

• You can't be serious!

  a fixed phrase for surprise

💡Here “can't” is not about ability (I can't swim); it means “this is impossible / not true”.

The same word, two meanings. Guessing usually goes with be, be + -ing or state verbs (know, have, like, live); rules usually go with action verbs — but always check the context. See also the lesson “Obligation: must / have to / need to”.

To guess about something that already happened, use must have + V3 and can't / couldn't have + V3. Don't use must be for the past. More in the lesson “Modal perfect”.

• The ground is wet. It must have rained last night.

• He can't have finished already — he only started ten minutes ago.

• ✗That mustn't be true.

  ✓That can't be true.

  Mustn't means “not allowed”; for a negative guess use can't.

• ✗He must is at home.

  ✓He must be at home.

  After must, use the base verb.

• ✗She must to be at work.

  ✓She must be at work.

  No to after must.

• ✗You can't being serious.

  ✓You can't be serious.

  After can't, use the base form be.

• ✗He must works here.

  ✓He must work here.

  No -s after must.

• ✗Lan didn't come yesterday. She must be sick.

  ✓Lan didn't come yesterday. She must have been sick.

  For a guess about the past, use must have + V3.