Modals (Modal Auxiliaries): can, may, must, shall, ought to — Uses and Rules
This page gives all the major modal auxiliaries — can, could, may, might, shall, should, will, would, must, ought to, need, dare and used to — their meanings (ability, permission, obligation, threat, etc.), rules and common mistakes.
- Class 6-10 grammar level
- 9 min read
What Are Modals and Their Rules
Modal auxiliaries are auxiliary verbs that come with the main verb to express the speaker's sense — ability, permission, possibility, obligation, duty, advice, threat, wish, and so on. The main modals are: can, could, may, might, shall, should, will, would, must, ought to; need, dare and used to are called semi-modals.
Main rules: (1) The first form of the verb (V1) follows a modal — He can swim, not He can swims. (2) Even in the third person singular, modals don't take -s — She must go. (3) 'ought to' and 'used to' include 'to'; the other modals are not followed by 'to' — You must to go is wrong. (4) Two modals don't go together — will can is wrong; instead write will be able to. (5) Modals have no -ing or V3 form. (6) do/does/did is not used to form questions and negatives — Can you help? / You must not smoke.
'Must' has no separate past form; 'had to' is used for past obligation — I had to walk home. Similarly, 'will/shall be able to' is used for a future ability of 'can', as in: I don't think I shall be able to go there.
Each Modal and Its Meaning
| Modal | Meaning | Example |
|---|---|---|
| can | ability, informal permission | She can speak three languages. / You can go now. |
| could | past ability, a polite request, a lower possibility | He could not catch the thief. / Could you open the window? |
| may | formal permission, possibility, a wish | May I come in, sir? / It may rain today. / May you live long! |
| might | a lower possibility, the past form of 'may' (in reported speech) | He might come, but I am not sure. |
| shall | future with I/We; with You/He/They, a command, promise or threat | We shall overcome. / You shall be punished for this. (threat) |
| should | advice, duty | You should see a doctor. / We should respect elders. |
| will | future with You/He/They; with I/We, firm resolve, a promise | I will help you. (promise) |
| would | a polite request, a past habit, a wish | Would you help me? / He would walk every morning. |
| must | obligation/necessity, a confident assumption | Staff must be at their desks by 9:00 am. / He must be tired. |
| ought to | moral obligation | You ought to pay your debts. / We ought to help the poor. |
| need | necessity — as a modal, usually in the negative/question | You need not worry. / Need I go? |
| dare | to have the courage — as a modal, usually in the negative/question | How dare you speak like that? / He dare not oppose. |
| used to | a past habit that no longer continues | I used to play cricket in my childhood. |
Subtle Differences Between Close Modals (These Trap You Most in the Exam)
| Pair | Difference | Example |
|---|---|---|
| must vs should | must = a mandatory rule/obligation; should = advice, with no penalty for not following it | You must wear a helmet (rule). You should eat fruits (advice). |
| should vs ought to | both express duty; 'ought to' lays more stress on moral/social obligation | You ought to obey your parents. |
| can vs may | can = ability; may = formal permission or possibility | I can lift this box. / May I use your pen? |
| may vs might | 'might' expresses a lower possibility than 'may' | It may rain. / It might rain (lower possibility). |
| must not vs need not | must not = prohibition; need not = not necessary, but not forbidden either | You must not park here. / You need not come tomorrow. |
| can vs could | 'could' = the past of 'can' (ability), or a more polite request | When I was five, I could swim. / Could I leave early? |
Modal + have + V3: Past-Time Meanings
The structure modal + have + V3 is used when expressing a guess, regret or possibility about a past event. must have + V3 = a confident guess about the past (The road is wet; it must have rained). should have / ought to have + V3 = regret or criticism about a duty that wasn't fulfilled (You should have told me earlier). may have / might have + V3 = a past possibility (He may have missed the bus). could have + V3 = the ability existed but the action didn't happen (You could have won the match).
How to identify the modal from the sense: if the sentence is a rule, a notice board or a law, use must; if it's a moral matter (repaying a debt, serving elders), use ought to; if it's advice, use should; if the sentence is in the past tense and about ability, use could; if there's a threat/command/promise with the second or third person, use shall. Most modal-based questions in the exam are solved using exactly these clues.
Memory Trick / Exam Tip
Remember the formula: 'rule = must, morality = ought to, advice = should, permission = may, ability = can (could in the past), threat/command (with You/He) = shall.' When the sense is given in brackets, pick the answer directly from this formula; and a modal is always followed by V1 — without 'to' (except for ought to and used to).
Frequently Asked Questions
How many modals are there?+
The main modals are can, could, may, might, shall, should, will, would, must and ought to. need, dare and used to are called semi-modals.
What is the difference between Must and should?+
'Must' expresses a mandatory obligation or rule (Staff must be at their desks by 9 am), while 'should' expresses only advice or duty (You should exercise daily).
When is 'Ought to' used?+
To express a moral or social duty, as in You ought to pay your debts. V1 follows it, and the 'to' is never dropped.
What is the past tense of 'Must'?+
'Must' has no separate past form. 'had to' is used for past obligation; 'must have + V3' is used for a past guess.