Modal Verbs

Modal verbs are special helping verbs that tell us about ability, permission, or obligation. You already know the basics -- can, cannot, may, must, and should. Now you will learn how to choose the correct modal in different situations and use polite forms like could and would.

What You'll Learn

• How to choose between can and may for ability and permission
• How to use must and should for obligation and advice
• How to use could and would for polite requests
• How to pick the correct modal to match the meaning of a sentence

When to Use

1. Talking about ability (can/cannot): "She can ride a bicycle without training wheels."
2. Asking for permission (may/can): "May I go to the washroom, Mrs Tan?"
3. Expressing obligation or a rule (must): "All students must return their library books by Friday."
4. Giving advice (should): "You should drink more water during recess."
5. Making polite requests (could/would): "Could you pass me the scissors, please?"

How to Form

Basic Structure

Modal verbs always come before the base form of the main verb. The main verb does not change -- no -s, -ed, or -ing.

Modals and Their Main Uses

Negative Forms

Key Rules

1. No "-s" for the third person: Modals never change form. Write "He can cook," not "He cans cook." This is different from ordinary verbs like "He cooks."

2. No "to" after can, may, must, should: The base verb follows directly. Write "She can swim," not "She can to swim." Write "You must finish," not "You must to finish."

3. "May" for permission, "can" for ability: In examinations and formal writing, use may when asking for permission ("May I borrow your ruler?") and can when talking about ability ("I can spell that word"). In everyday speech, people also use "can" for permission, but the formal rule is often tested.

4. "Must" vs "should": Must is stronger than should. Use "must" for rules and things that are compulsory ("You must wear your uniform"). Use "should" for advice or things that are a good idea ("You should eat your vegetables").

5. "Must not" means forbidden, not optional: "You must not copy your friend's work" means it is not allowed at all. Do not confuse this with "do not have to," which means it is not necessary.

6. "Could" and "would" for politeness: Use could or would to make requests sound more polite. "Could you open the window?" is more polite than "Can you open the window?"

Common Mistakes

Clue Words

Permission clues (may, can)

allowed, permitted, let, may I, excuse me

Ability clues (can, cannot)

able to, manage to, know how to, good at

Obligation / rule clues (must)

required, compulsory, must, have to, rule, not allowed

Advice clues (should)

better, wise, advisable, a good idea, recommend, ought to

Polite request clues (could, would)

please, kindly, would you mind, excuse me

Tip: When choosing between can and may, ask yourself: "Is the sentence about ability (what someone is able to do) or about permission (what someone is allowed to do)?" Ability = can. Permission = may.

Practice Tips

1. The ability-or-permission test: Read the sentence and decide whether it is about what someone is able to do or what someone is allowed to do. If it is ability, choose "can." If it is permission, choose "may."
2. The strength test: Decide how strong the sentence needs to be. Is it a rule (must)? Is it just advice (should)? Is it a polite request (could/would)? Matching the modal to the strength of the message is the key.
3. The base-verb check: After writing a sentence with a modal, look at the main verb. It should be in its base form -- no -s, no -ed, no -ing. If it has any of these endings, remove them.
4. The "must not" trap: When you see a sentence about something that is optional or not needed, never choose "must not." That means it is forbidden. Instead, use "do not have to."

Quick Reference