Intermediate Grammar
Master English grammar one unit at a time. Clear notes, then 40+ interactive exercises with instant feedback — based on the topics in English Grammar in Use (Intermediate).
Must and can’t“must be / can’t be · must have done / can’t have done” — 44 interactive questions
📖 Grammar Reference — Must and can’t
Study the notes, then work through the six exercises.
A must — I’m sure it’s true (now)
Use must + base verb when you look at the evidence and feel sure something is true. You are not stating a fact — you are making a confident guess (a deduction).
| must be | You’ve worked all day. You must be exhausted. |
| must be doing | The light’s on, so she must be working late. |
| must + verb | He speaks four languages — he must have a good ear. |
💡 After must use the base verb — never to: (not must to be).
B can’t — I’m sure it’s NOT true (now)
The opposite of must is can’t (not mustn’t). Use can’t when you feel sure something is impossible or untrue.
C must have (done) — I’m sure it happened
For a sure guess about the past, use must have + past participle. You are sure something happened, because of what you can see now.
💡 Say must have, never must of. They sound the same in speech, but of is wrong in writing.
D can’t have (done) — I’m sure it didn’t happen
For a sure guess that something did not happen, use can’t have (or couldn’t have) + past participle.
Made with care for English learners · allenglish4u.com
Original practice material inspired by the English Grammar in Use syllabus (Cambridge University Press). Example sentences and exercises are written by All English 4U.