Modal Verbs

You already know the basic modals -- can, may, must, should -- and their expanded forms like could, might, would, shall, ought to, and have to. At the mastery level, you will learn to choose the right modal by thinking about the function it serves: giving advice, expressing obligation, describing possibility, or making a deduction.

What You'll Learn

• How to select the most appropriate modal when expressing advice (should, ought to, could, had better)
• How to distinguish between obligation (must, have to) and prohibition (must not) vs lack of obligation (do not have to)
• How to express different degrees of possibility (could, may, might, can)
• How to use modals for deduction -- drawing conclusions about what is likely, possible, or certain (must, can't, could, may, might)

When to Use

1. Giving advice or recommendations: "You should apologise to your friend for what you said." / "You ought to revise early rather than wait until the night before the PSLE."
2. Expressing strong obligation or duty: "All passengers must fasten their seatbelts before the plane takes off." / "We have to submit our project by Friday."
3. Talking about lack of obligation: "You do not have to wear your school uniform on Saturday -- it is a free-dress day."
4. Describing possibility or uncertainty: "It might rain later, so you could bring an umbrella just in case."
5. Making a deduction based on evidence: "The lights in the classroom are off. The students must have gone home." / "She answered every question correctly. She can't be new to this topic."

How to Form

Modal + Base Verb (No Exceptions)

Every modal verb is followed by the base form of the main verb -- no -s, -ing, or -ed.

Modals Grouped by Function

Negative Forms

Key Rules

1. "Must not" vs "do not have to" -- they are opposites: "Must not" means it is forbidden ("You must not litter"). "Do not have to" means it is optional ("You do not have to bring food -- lunch is provided"). Confusing these two is one of the most common errors at the PSLE level.

2. "Should" vs "had better" -- degree of urgency: Both give advice, but "had better" carries a warning. "You should study for your test" is a recommendation. "You had better study for your test" implies something bad will happen if you do not. Note: "had better" is always followed by the base verb, never "had better to".

3. "Must" for deduction requires evidence: When "must" is used to mean "I am almost certain," there should be a reason. "He is yawning and rubbing his eyes -- he must be tired." Without evidence, "must" sounds like an obligation instead.

4. "Could" has three functions: (a) Past ability -- "When I was younger, I could run very fast." (b) Polite request -- "Could you pass the salt?" (c) Present possibility -- "The answer could be C." Use context to determine the meaning.

5. "May" vs "might" -- slight difference in likelihood: Both express possibility, but "may" suggests a slightly higher chance than "might." In practice, they are often used interchangeably in primary school English, but in the PSLE, "might" is the safer choice when something is less likely.

6. "Ought to" behaves differently from other modals: Unlike most modals, "ought" is always followed by to + base verb. "She ought to help her classmates." The negative is "ought not to" (not "ought to not"). In Singapore English, "ought to" is more formal than "should" and appears less frequently in everyday speech, but it does appear in examinations.

7. Use "have to" for external rules, "must" for personal conviction: "We have to wear masks in the hospital" (the hospital's rule). "I must remember to call my grandmother" (the speaker's own sense of duty). This distinction is subtle but tested in higher-order questions.

Common Mistakes

Clue Words

Advice (should, ought to, had better, could):

recommend, suggest, advise, it would be wise, it is a good idea, otherwise (signals "had better")

Obligation (must, have to):

required, compulsory, necessary, rule, regulation, law, duty, expected to

No obligation (do not have to, need not):

optional, not necessary, not compulsory, free to choose, up to you

Prohibition (must not, cannot):

forbidden, not allowed, banned, against the rules, strictly prohibited

Possibility (could, may, might):

perhaps, maybe, possibly, it is possible that, there is a chance

Deduction (must, can't):

clearly, obviously, certainly, no doubt, there is no way (for can't), evidence suggests

Tip: When choosing between modals in a fill-in-the-blank question, first identify the function -- is the sentence about advice, obligation, possibility, or deduction? Then pick the modal that matches both the function and the strength needed.

Practice Tips

1. The function-first strategy: Before selecting a modal, label the sentence's purpose. Write "A" for advice, "O" for obligation, "P" for possibility, or "D" for deduction in the margin. Then choose from the modals that serve that function.
2. The substitution test for "must not" vs "do not have to": Replace the modal phrase with "it is forbidden" or "it is optional." If "it is forbidden" makes sense, use "must not." If "it is optional" fits, use "do not have to."
3. Look for evidence when choosing deduction modals: If the sentence gives a reason or observation (such as "The bag is very heavy"), a deduction modal like "must" or "can't" is likely correct. If no evidence is given, the sentence is probably about obligation or advice instead.
4. Check for "to" after the modal: Most modals are followed directly by the base verb. Only "ought to" and "have to" include "to." If you see "should to" or "must to," remove the "to."

Quick Reference