The Grammaring Guide to English Grammar with Exercises

Modals to express permission / prohibition: CAN, COULD, MAY, MIGHT, BE ALLOWED TO, MUSTN'T, BE TO

Present and future

Can and may are used to give permission. May is more formal and less common in everyday contexts:

You

can borrow

my car if you want.

Children

may use

the pool with adult supervision.

The negative forms cannot/can't, mustn't and may not (more formal) are used to express prohibitions:

I'm sorry, but you

can't stay

here.

You

mustn't touch

that Ming vase.

Non-committee members

may not vote

on committee issues.

Used in the negative, be to can also express prohibitions:

You

are not to do

that again!

Can, could and may are used to ask for permission. Can is the most direct of the three, could is more formal and tentative, and may is the most formal:

Can

I

use

your phone?

Could

I

have

a glass of water?

May

I

start

my presentation?

Might, which has a more tentative meaning, is very formal and is rarely used:

Might

I

ask

you a question?

When we talk about permission but we are not actually giving, refusing or asking for it, we can use can/can't or be allowed to:

Students

can choose

any topic for their project.

We

can't eat

in the library.

Are

we

allowed to smoke

in this restaurant?

Will

journalists

be allowed to ask

questions?

Past

Could can be used to express general permission in the past:

When I was a child, I

could do

basically anything that I wanted to do.

But when we talk about a particular action which was permitted and performed, we use was/were allowed to:

Although I was underage, I

was allowed to enter

the race.