Modal verbs: can, must, should and what sits between them
Six words that change a sentence more than any tense does — and the negative that catches everyone.
Modal verbs are not about time. They are about stance: I can, I must, I should, I might. There are few of them, they do not change their form, and they are what makes speech sound adult.
One rule: they all behave the same
The good news first. Every modal:
- takes no `-s` in the third person: she can, never she cans
- takes no `to` after it: I can swim, never I can to swim
- needs no `do` for questions and negatives: Can you?, I can't
The one exception is have to, which behaves like an ordinary verb: she has to, Do you have to?
Six of them, and their jobs
| Word | Meaning | Example |
|---|---|---|
can | ability, permission | I can drive. |
could | past ability; polite request | Could you help me? |
must | obligation you set yourself | I must call her. |
have to | obligation circumstances set | I have to work on Saturday. |
should | advice | You should sleep more. |
might | possibility, either way | It might rain. |
The distinction people get wrong
`must` against `have to`. Both mean an obligation, but they come from different places:
- I must stop smoking — I decided this.
- I have to work on Saturday — my boss decided this.
In the negative they separate completely:
- You mustn't go — forbidden.
- You don't have to go — not required, but you may.
This is the most expensive mistake on the list. Saying "you don't have to" when you mean "you are forbidden to" leads somewhere entirely different.
`can` against `may`. In speech can covers both ability and permission. may is formal and belongs in writing.
`should` against `must`. should is advice you may ignore. must is not.
Politeness is built out of them
A direct request sounds blunt in English, and modals are what soften it:
- Give me the salt. — nearly an order
- Can you give me the salt? — fine
- Could you give me the salt? — polite
- Would you mind giving me the salt? — very polite
One word changes the whole register. That is why modals are better learned as tone than as grammar.
About the past
Modals have no ordinary past tense. Constructions stand in for one:
- I could swim — I was able to, as a child
- I had to go — I was obliged to
- I should have called — I ought to have, and did not
The last one is regret, and it turns up in conversation constantly.
Where this sits in the course
In Lymi the modals come as a block: can and could in lesson thirty-one, must and its negatives in thirty-two, should in thirty-three, have to in thirty-four. Consecutively, and spoken aloud — because the gap between "forbidden" and "not required" only settles through your own speech, never through a table.