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:

• talk about possibility and ability
• make requests
• ask for or give permission

Structure of can

The basic structure for can is:

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

Notice that:

• Can is invariable. There is only one form: can
• The  main verb is always the bare infinitive.

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:

• She can  drive a car.
• John can  speak Spanish.
• I cannot  hear you. (I can't  hear you.)
• Can  you hear me?

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  you make a cup of coffee, please.
• Can  you put the TV on.
• Can  you come here a minute.
• Can  you be quiet!

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:

• talk about past possibility or ability
• make requests

Structure of could

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

Notice that:

• Could is invariable. There is only one form: could
• The  main verb is always the bare infinitive.

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:

• I could  swim when I was 5 years old.
• My grandmother could  speak seven languages.
• When we arrived home, we could not open the door. (...couldn't  open the door.)
• Could  you understand what he was saying?

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:

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):

• Could  you tell me where the bank is, please?
• Could  you send me a catalogue, please?

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:

• to talk about ability

Structure of be able to

The basic structure for be able to is:

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

• I  was able to drive...
• I  will be able to drive...
• I  have been able to drive...

Notice too that be able to has an infinitive form:

• I would like to be able to speak Chinese.

Use of be able to

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:

• I  have been able to swim since I was five. (present perfect)
• You  will be able to speak perfect English very soon. (future simple)
• I would like to be able to fly an airplane. (infinitive)

sources : Original Link