The Rule
| Modal | Main use | Example |
|---|---|---|
| can | Ability (present) / permission (informal) | I can speak three languages fluently. |
| could | Ability (past) / polite request / weak possibility | Could you send me the file? / She could be in a meeting. |
| should | Advice / recommendation / mild obligation | You should speak to your manager directly. |
| must | Strong obligation / logical deduction | You must submit the form. / He must be exhausted. |
| might / may | Possibility (less certain than must) | She might be late. / It may be delayed. |
| have to | External obligation — rule or requirement | I have to submit expenses by Friday. |
| don't have to | No obligation — not necessary but allowed | You don't have to attend — it is optional. |
| mustn't | Prohibition — not allowed | You mustn't share your password. |
Critical difference:
You don't have to wear a tie. = It is not necessary — but you can if you want.
You mustn't be late. = It is forbidden — absolutely not allowed.
All modal verbs are followed by the bare infinitive — no 'to':
✗ You must to finish the report. ✓ You must finish the report.
You don't have to wear a tie. = It is not necessary — but you can if you want.
You mustn't be late. = It is forbidden — absolutely not allowed.
All modal verbs are followed by the bare infinitive — no 'to':
✗ You must to finish the report. ✓ You must finish the report.
Common Mistakes
You must to finish the full report by today at noon.
You must finish the full report by today at noon. (no 'to')
She can to speak four languages including Japanese.
She can speak four languages including Japanese. (no 'to')
I don't must work on Sundays — it is not in my contract.
I don't have to work on Sundays — it is not in my contract.
He should to call the client before the end of the day.
He should call the client before the end of the day. (no 'to')
Gap Fill — Press Enter to Check
Error Correction
Multiple Choice
Put the Words in the Correct Order
Sentence Transformation — Press Enter to Check
Conversation Practice
1
What must you do in your job every week? What do you not have to do? What are you not allowed to do?
2
Give your partner 3 pieces of professional advice using 'should', 'shouldn't', and 'ought to'.
3
Describe something uncertain that might happen at your company this year. Use might, could, and may.
Homework
Write a short policy document (8 sentences) for new employees at your company. Use must, mustn't, have to, don't have to, should, and can — at least once each.