GRAMMAR LESSON
Modal Verbs – Possibility, Permission, Advice, and Obligation
UNDERSTAND THE PATTERN
The rule in plain English
Core modal verbs such as can, may, might, should and must come before a base verb: she can swim. They express meanings such as ability, permission, possibility, advice and obligation. Related expressions have different patterns: ought to go and need to go.
SEE HOW IT WORKS
Examples in context
- Can I use your phone? — permission
- You must wear a seatbelt. — obligation
- I should have called earlier. — past regret
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Core modal verbs such as can, may, might, should and must come before a base verb: she can swim. They express meanings such as ability, permission, possibility, advice and obligation. Related expressions have different patterns: ought to go and need to go.
Compare: Can I use your phone? — permission
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Use modals to express a viewpoint
Modals show meanings such as possibility, obligation, advice and willingness. The core modals use a following base verb and do not add -s: She might arrive, not she might arrives. A modal form does not always identify one time period: could may refer to past ability, present possibility or a polite request.
Possibility and deduction
May, might and could can all express uncertain possibility: The delivery might arrive tomorrow. Their interpretation depends on context. Do not assign them fixed percentages or assume that might always expresses less likelihood than may.
Must can express a strong conclusion from evidence: The lights are on and her bicycle is outside; she must be home. Can’t can express a conclusion that something is impossible: That can’t be the final version; it still contains yesterday’s missing sections. These are the speaker’s deductions, not guarantees.
Obligation and permission
Must and have to both express obligation. Must often presents the speaker’s requirement, while have to often describes an external requirement, but this is not an absolute division. For a past obligation, use had to: Yesterday we had to leave early. There is no modal form musted.
Mustn’t means something is prohibited. Don’t have to means it is unnecessary: You mustn’t enter this room differs from You don’t have to enter this room.
Looking back with modal + have
- Must have + past participle: She must have missed the message. A strong deduction about the past.
- Might have: She might have taken a different bus. An uncertain past possibility.
- Could have: We could have booked earlier. A past opportunity; context can also make could have a guess about what happened.
- Should have: I should have checked the address. Advice viewed retrospectively, often with regret.
For other forms of modal ability can, use be able to: I hope to be able to join you. This concerns modal can; the separate verb meaning preserve food does have forms such as canning.
Quick self-check
- Express past obligation: Yesterday, we ___ leave at six. Use have to.
- Which means no obligation: mustn’t or don’t have to?
- Complete the strong past deduction: She must ___ forgotten. Use have/has.
Check the answers
1. Had to. 2. Don’t have to. Mustn’t prohibits the action. 3. Have. After must, use have + forgotten.
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