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can, could, be able to

can and could are modal auxiliary verbs. be able to is NOT an auxiliary verb (it uses the verb be as a main verb). We include be able to here for convenience.

In this lesson we look at can, could and be able to, followed by a quiz to check your understanding.

can

Can is an auxiliary verb, a modal auxiliary verb. We use can to:

Structure of can

The basic structure for can is:

subject + auxiliary verb
can
+ main verb

The main verb is always the bare infinitive (infinitive without to).

subject auxiliary verb
can
main verb  
+ I can play tennis.
- He cannot play tennis.
can't
? Can you play tennis?

Notice that:

The main verb is always the bare infinitive (infinitive without to). We cannot say: I can to play tennis.

Use of can

can for possibility and ability

We use can to talk about what is possible, what we are able or free to do:

Normally, we use can for the present. But it is possible to use can when we make present decisions about future ability.

  1. Can you help me with my homework? (present)
  2. Sorry. I'm busy today. But I can help you tomorrow. (future)

can for requests and orders

We often use can in a question to ask somebody to do something. This is not a real question - we do not really want to know if the person is able to do something, we want them to do it! The use of can in this way is informal (mainly between friends and family):

can for permission

We sometimes use can to ask or give permission for something:

  1. Can I smoke in this room?
  2. You can't smoke here, but you can smoke in the garden.

(Note that we also use could, may, might for permission. The use of can for permission is informal.)

could

Could is an auxiliary verb, a modal auxiliary verb. We use could to:

Structure of could

subject + auxiliary verb
could
+ main verb

The main verb is always the bare infinitive (infinitive without to).

  subject auxiliary verb
could
main verb
+ My grandmother could swim.
- She could not walk.
couldn't
? Could your grandmother swim?

Notice that:

The main verb is always the bare infinitive (infinitive without to). We cannot say: I could to play tennis.

Use of could

could for past possibility or ability

We use could to talk about what was possible in the past, what we were able or free to do:

We use I (positive) and couldn't (negative) for general ability in the past. But when we talk about one special occasion in the past, we use be able to (positive) and couldn't (negative). Look at these examples:

past
general specific occasion
+ My grandmother could speak Spanish. A man fell into the river yesterday. The police were able to save him.
- My grandmother couldn't speak Spanish. A man fell into the river yesterday. The police couldn't save him.

could for requests

We often use could in a question to ask somebody to do something. The use of could in this way is fairly polite (formal):

be able to

Although we look at be able to here, it is not a modal verb. It is simply the verb be plus an adjective (able) followed by the infinitive. We look at be able to here because we sometimes use it instead of can and could.

We use be able to:

Structure of be able to

The basic structure for be able to is:

subject + be + able + to-infinitive
  subject main verb
be
adjective
able
to-infinitive
+ I am able to drive.
- She is not able to drive.
isn't
? Are you able to drive?

Notice that be able to is possible in all tenses, for example:

Notice too that be able to has an infinitive form:

Use of be able to

Be able to is NOT a modal auxiliary verb. We include it here for convenience, because it is often used like "can" and "could", which are modal auxiliary verbs.

be able to for ability

We use be able to to express ability. "Able" is an adjective meaning: having the power, skill or means to do something. If we say "I am able to swim", it is like saying "I can swim". We sometimes use be able to instead of "can" or "could" for ability. Be able to is possible in all tenses - but "can" is possible only in the present and "could" is possible only in the past for ability. In addition, "can" and "could" have no infinitive form. So we use be able to when we want to use other tenses or the infinitive. Look at these examples: