Can, Could, May, Must, Have to comparison
B1 IntermediateA comparison of modal verbs used to express ability, permission, obligation, and possibility. This guide clarifies the differences in formality and strength between can, could, may, must, and have to.
Overview of Modals
Modal verbs change the meaning of the main verb to show how certain, allowed, or necessary an action is. While some overlap, they differ in formality and intensity.
- Can/Could: Used for ability and requests.
- May: Used for formal permission and possibility.
- Must/Have to: Used for strong obligation and necessity.
Permission and Requests
Use could or may when speaking to a stranger or in a professional setting to sound more respectful.
| Modal | Usage | Formality |
|---|---|---|
| Can | General permission and informal requests | Informal |
| Could | Polite requests or past ability | Neutral / Polite |
| May | Formal permission or high-level requests | Formal |
Obligation: Must vs. Have to
In the negative, the meanings change significantly: mustn't means it is forbidden, while don't have to means it is not necessary but optional.
| Modal | Source of Obligation | Example |
|---|---|---|
| Must | Internal (speaker's feelings or authority) | I must finish this book. |
| Have to | External (rules, laws, or circumstances) | I have to wear a uniform at work. |
Grammar Rules
Subject + Modal + Base Verb (e.g., I can go)
Modal verbs do not change for the third person (no -s) and are followed by the base form of the verb without 'to'. The exception is have to, which functions like a normal verb.
Common mistakes
I must to go now.
I must go now.
Do not use 'to' after must, can, could, or may.
He cans swim very well.
He can swim very well.
Modal verbs do not take an -s in the third person singular.
You mustn't come if you are tired.
You don't have to come if you are tired.
Use 'don't have to' for lack of necessity. 'Mustn't' indicates a prohibition.
Practice
Choose the most polite option: '___ I borrow your pen, please?'
Show answer
May
Answer:May
Why:'May' is the most formal and polite way to ask for permission in this context.
Complete the sentence for a legal requirement: 'Drivers ___ stop at a red light.'
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have to
Answer:have to
Why:'Have to' is preferred for external rules and laws, though 'must' is also grammatically possible for strong obligations.
Correct the error: 'She coulds speak three languages when she was ten.'
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She could speak three languages when she was ten.
Answer:She could speak three languages when she was ten.
Why:Modal verbs like 'could' never take an -s ending, regardless of the subject.