Quick introduction to modal verbs

The normal structure of a modal verb sentence is modal verb + infinitive.

I'm not good at picking colors, I may pick the wrong one.

She said she must call her sister.

For negative sentences it's modal verb + not + infinitive. Be aware that may and not always appear separate.

You shouldn't have said that.

She may not arrive in time.

The main distinction between true modal verbs such as can, could, should, may, might and other verbal forms (such as have to and be able to) which serve the same purpose but function like normal verbs, is that modal verbs are invariable (i.e. their form never changes) and this often restricts their usage to particular tenses. It's therefore helpful to know alternatives and always understand context.

We'll group modal verbs and their equivalents according to the context in which they can be used.

Obligation

have to, need to, must

To describe an action that is necessary or an obligation, you can use have to, need to or must. These verbs have very similar meanings and can be used interchangeably in most circumstances.

All passengers must wear their seatbelt at all times during the flight.

You need to be more careful next time.

I had to stay in bed for a week when I got sick.

I'll have to get up early tomorrow.

Have to is mostly used for general or external obligations. In spoken English, it's common to say (have) got to instead.

I have to be on time for work.

"Not now, I've got to go!", she said.

Need to is related to necessities or specific requirements.

I need to find a doctor.

You need to fill out this application.

Must is often associated with personal and moral obligations. Please note that must doesn't have a past form.

You must keep this a secret.

I must fulfill my duty to my country.

Should can be used to express obligation in a more polite or less forceful way:

You should finish your report before the end of the month.

No obligation

don't have to, don't need to

The negative forms of have to and need to are don't/doesn't have to and don't/doesn't need to(other tenses are formed regularly). They express the meaning that something is not necessary.

You don't have to stay if you don't want to.

Rich people don't need to work.

I didn't have to to wake up so early today, but I've got used to it.

Keep in mind that we don't use haven't got to express obligation, only possession. The correct form is don't have.

You don't have to stay. NOT You haven't got to stay.

Possibility and ability

can, could, be able to

A sentence using can is open to various interpretations depending on the context:

I can sing that song. (it's possible that may I sing that song, but I may also sing/do something else)

I can sing that song. (I know how to sing that song)

I can sing that song. (I have permission to sing that song.)

In the present and the future, both can and could are correct. In the past, use could only.

I could call her and ask her if she's OK. (present)

I could speak German as a child, but it's hard for me to do it now. (past)

Related to its use in the past, could is common in 2nd and 3rd type conditionals which express unlikely or imaginary situations. This condition is often not explicit, only implied.

Can you come at 7?

I could, if I didn't have so much work to do.

Do you see the screen?

I could... (if you moved away).

If we want to express notions like ability, capacity and availability, an alternative is be able to. It's more formal than can/could.

I'm able to meet you on Wednesday.

We can use be able to in the present, past, future, as a gerund or infinitive, allowing more flexibility than modals like can/could, which are invariable.

I'll finally be able to get some rest.

I'd like to be able to visit my family in Spain.

Possibility and ability (negative forms)

can't, couldn't, (not) be able to

The negative form of can is can't (or cannot for emphasis). Just like its affirmative counterpart, the precise meaning depends on the context.

I can't sing that song. (it's not possible for me to sing that song because I don't want to, it's inappropriate, etc.)

I can't sing that song. (I don't know how to sing it)

I can't sing that song. (I'm not allowed to sing it)

Always use couldn't in the past, never in the present.

I couldn't come to class yesterday, the snow was blocking all the roads.

Just like be able to is related to ability, (not) be able to expresses inability.

He said he won't be able to meet the deadline

I wasn't able to hear what she said.

Permission and prohibition

can/can't, could/couldn't, mustn't

Can and its negative can't(or cannot for emphasis), as we've seen before, are also used to express permission and prohibition, respectively. Another expression is be allowed/(not) be allowed, which is more formal.

Can you park on this spot? OR Are you allowed to park on this spot ?

You can't smoke inside the restaurant. OR You're not allowed to smoke inside the restaurant.

Both can and may are used to to ask and give permission, with may being more formal. Depending on the context, this can function as a request, an invitation or an offer of services.

Can/May I open this window? (permission)

You can/may speak. (permission)

Can you hand me that small box over there? (request)

Can/may I buy you a drink? (invitation)

Can/May I help you? (service)

To express prohibition, we also have must not/mustn't.

I mustn't fail.

You mustn't tell our mother what happened.

Advice and opinion

should/shouldn't

Should is not as strong as must or have to/need to. It's used to say we think something is right.

You should study more if you want good grades.

I should be working, instead I'm here listening to you.

In negative sentences, it becomes shouldn't.

He realized he shouldn't have done that.

As we've seen elsewhere, should/shouldn't can be used as a softer way of expressing an obligation or prohibition.

The doctor told me I should exercise more.

You shouldn't be here.

Ought to is a slightly stronger alternative to should. The negative form is ought not to. Its meaning is close to must when used for strong suggestions .

You ought to spend your money more carefully.

You ought to/must see that movie. It's fantastic!

Probability

must, may, might, can't

Probability is expressed in degrees. The formulas used for obligations (see above) can be used when we are certain something is true. These are must and have to(but not need to):

The man must be hiding in that room.

You have to be joking!

When something is only possibly true, use may/might:

She might be the suspect we've been looking for.

This may take a few minutes.

The negative forms are may not /might not:

We may not arrive in time.

We might not have noticed you if you hadn't taken off your hat.

Don't confuse the modal verb may in "may be" with the adverb maybe. They are used in different ways:

You may be ill, your face looks pale.

Your face looks pale, maybe you're ill.

When something is impossible, use can't(in the present or future) or couldn't( in the past):

They can't be the same person, his face looks too different.

Our team has the best players, we can't lose.

He couldn't have been in two places at the same time.