Explanation on the use of modal verbs in English

1. Modal verbs

2. Modal verbs are special verbs. They don’t
usually need an auxiliar as they usually act
as auxiliaries, they go before the verb , they
don’t have all their forms and they can
express different things: ability, permission,
prohibition, possibility or probability,
deduction and certainty.

3. Ability
To express ability we use can
I can ride a bike
I can’t drive a car
The past and conditional forms of can are
could
When I was young I could jump over that
fence
The infinitive form of can is to be able to,
so we say
I would like to be able to fly
I have never been able to dance

4. Permission
• In questions I can use can or may, the second is
more formal than the first
May I use your phone, please?
• In general we use can to express permission, so
sometimes it’s difficult to know whether we are
refering to ability or permission.
I can ride a motorbike, now I have the licence
I can go out at nights, my parents allow me to.
• When can means permission the infinitive form is
to be allowed to, so I say:
When I was young I was not allowed to go out on my
own
I will never be allowed to quit my judo classes

5. Obligation and prohibition
• In the affirmative, we use have to and must to
express obligation but only in the present as must
doesn’t have a past form
I have to do my homework
I must do the washing-up
In the past I had to do many things in the house
• To express lack of obligation we use the negative
form of have to or need to
I don’t have to make my bed
• To express prohibition we use mustn’t or may not
You mustn’t touch this switch

6. Advice
• To express advice we use should or ought to, both
in the affirmative and negative forms.
You should visit a doctor if you are not feeling well
You shouldnt have gone to the park today, it’s been
raining all day and you’ll catch a cold
• Ought to is less usual than should but we use it in
the same way
You ought to have told me about it before, I didn’t know
he was ill
He oughtn’t to go out at night so often, he’s going to get
ill

7. Deduction and certainty
• We use must to express deduction in the
affirmative
Look at his golden watch, he must be very rich
• And we use can’t in the negative
He can’t be very kind, look at the expression in his eyes
• When we are not so sure we can use may or
might
• He may be a doctor, he seems to be in charge
• She might be an actress, she’s really good-looking

8. Possibility or probability
• We use may and might to express possibility.
• When we are one hundred per cent
something will happen we use will
Tomorrow it will rain
• When we are not so sure, we say
Tomorrow it may rain
• And when it’s even less likely
It might rain tomorrow