Modal Verbs
Modal verbs are special helping verbs that add meaning to the main verb in a sentence. They tell us about ability, possibility, permission, obligation, or likelihood. You already know the basics -- can, may, must, and should. Now let's explore the full range of modals and learn their past forms and finer shades of meaning.
What You'll Learn
- How to use modal pairs: can/could, may/might, shall/should, will/would
- When to use must, have to, and ought to for obligation and advice
- How modals express different degrees of certainty, from strong to weak
- How to choose the right modal for formal and informal situations
When to Use
- Expressing ability (can/could): "She can speak three languages fluently." / "He could already swim when he was five."
- Asking for or giving permission (may/can/could): "May I borrow your reference book?" / "You may leave when you have finished."
- Talking about possibility (may/might/could): "It might rain later this afternoon, so bring an umbrella." / "The bus could be delayed due to roadworks."
- Giving advice or expressing duty (should/ought to): "You should revise your composition before handing it in." / "We ought to respect our elders."
- Expressing strong obligation or necessity (must/have to): "All students must wear the school uniform." / "We have to return our library books by Friday."
How to Form
Structure
Modal verbs follow a consistent pattern. The main verb after a modal is always in its base form (infinitive without "to").
| Structure | Example |
|---|---|
| Subject + modal + base verb | She can swim. |
| Subject + modal + not + base verb | He should not waste food. |
| Modal + subject + base verb? | Could you help me? |
Modal Pairs and Their Uses
| Modal | Past/Softer Form | Main Uses |
|---|---|---|
| can | could | ability, permission, possibility |
| may | might | permission (formal), possibility |
| shall | should | offers/suggestions, advice/duty |
| will | would | future actions, polite requests, preferences |
Degrees of Certainty
Modals express how sure you are that something will happen. Stronger modals show greater certainty.
| Certainty Level | Modal | Example |
|---|---|---|
| Very sure | will / must | "It will rain tonight." / "He must be at home -- the lights are on." |
| Fairly sure | should / ought to | "She should arrive by noon." |
| Possible | may / can | "The concert may be sold out." |
| Less sure | might / could | "They might change the schedule." |
Key Rules
-
No "to" after most modals: Write "She can swim," not
"She can to swim."The exceptions are ought to and have to, which always include "to." -
No "-s" for third person: Modals do not change form. Write "He can cook," not
"He cans cook."This differs from ordinary verbs like "He cooks." -
"Must" vs "have to": Both express obligation, but must comes from the speaker ("You must apologise") while have to comes from an external rule ("We have to wear our PE attire on Fridays"). In the negative, must not means it is forbidden, while do not have to means it is not necessary.
-
"Can" vs "may" for permission: In formal writing and school examinations, use may for permission ("May I go to the washroom?"). Use can for ability ("I can carry this box"). In everyday speech, "can" is also used for permission, but the formal distinction is frequently tested.
-
"Should" vs "ought to": Both give advice, but ought to is slightly more formal. "You should eat more vegetables" and "You ought to eat more vegetables" mean the same thing. In negative form, use "should not" (not
"ought to not"). -
"Could" for past ability vs present possibility: "When I was younger, I could run very fast" (past ability). "It could rain tomorrow" (present possibility). Context determines the meaning.
Common Mistakes
| Wrong | Right | Why |
|---|---|---|
| She can to play the piano. | She can play the piano. | No "to" after can |
| He musts finish his homework. | He must finish his homework. | Modals never take "-s" |
| You must not to litter. | You must not litter. | No "to" after must |
| I don't must go to school on Saturday. | I do not have to go to school on Saturday. | Use "do not have to" for lack of obligation; "must not" means forbidden |
| Can I go to the toilet? | May I go to the toilet? | Use "may" for permission in formal/exam contexts |
| She ought not to forgets her bag. | She ought not to forget her bag. | Base form after modal; no "-s" |
Clue Words
Permission clues
allowed, permitted, may, let
Ability clues
able to, capable, manage to, can
Obligation/necessity clues
required, compulsory, necessary, must, need to
Advice clues
better, advisable, wise, should, ought to
Possibility clues
perhaps, maybe, possibly, likely, might, could
Tip: Think of modals on a "certainty scale." Will and must sit at the top (very sure). Should and ought to sit in the middle. May, might, and could sit at the bottom (less sure). Matching the right modal to the right level of certainty is the key to scoring well.
Practice Tips
-
Substitution test: Replace one modal with another and check if the meaning changes. "You must attend" (compulsory) vs "You should attend" (advised). If the meaning shifts, you know you have the right modal.
-
Formal vs informal check: When answering exam questions, pick the more formal modal. Choose may over can for permission, and shall over will for offers ("Shall I help you?").
-
Negative meaning test: Watch out for must not (forbidden) vs do not have to (not necessary). Ask yourself: "Is it banned, or is it just optional?" This distinction is one of the most commonly tested points.
-
Context scan: Read the full sentence before choosing. Look for clue words like "compulsory" (must/have to), "perhaps" (may/might), or "advisable" (should/ought to) to guide your choice.
Quick Reference
| Modal | Positive Meaning | Negative Form | Negative Meaning | Example |
|---|---|---|---|---|
| can | ability, permission | cannot / can't | inability, not allowed | She can solve the puzzle. |
| could | past ability, possibility | could not | past inability | He could read before Primary 1. |
| may | permission, possibility | may not | not permitted, unlikely | You may begin the test now. |
| might | weaker possibility | might not | unlikely | We might visit the museum this weekend. |
| shall | offer, suggestion | shall not | prohibition (formal) | Shall we take the MRT? |
| should | advice, duty | should not | advised against | You should proofread your essay. |
| will | future, certainty | will not / won't | refusal, negative future | The ceremony will start at nine. |
| would | polite request, preference | would not | refusal (polite) | Would you pass me the dictionary? |
| must | strong obligation, certainty | must not | forbidden | You must hand in your homework on time. |
| have to | external obligation | do not have to | not necessary | We have to queue at the hawker centre. |
| ought to | advice (formal) | ought not to | advised against (formal) | You ought to arrive early for assembly. |