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 these three verbs, 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

subject + can + main verb

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

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

Notice that:

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

The main verb is always the bare infinitive (infinitive without "to"). We cannot say:

Use of Can

can: 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.

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

can: 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: Permission

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

Can I smoke in this room?
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

subject + could + main verb

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

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

Notice that:

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

The main verb is always the bare infinitive. We cannot say:

Use of Could

could: 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 could (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: 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 structure of be able to is:

subject + be + able + infinitive

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

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 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: 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)

Can, Could, Be able to Quiz

1	he understand what you were talking about?
2	My sister play tennis now.
3	I walk when I was less than a year old.
4	(Polite) you tell me what time it is, please?
5	My grandfather walk without any help last night.
6	I would like to play the piano.
7	How long have you drive?
8	I'll help you later.
9	Can you help me? I never understand this.
10	Will Man live forever one day?

Can, Could, Be able to

Can