Modal verbs of ability

Modals

1. Modals of ability

These modal verbs are used to express the ability or possibility that someone has to perform an action. These verbs are: can, could and be able to.

Samuel can speak four languages.

I wasn't able to finish my exam.

2. How are can and could used?

Can is used for actions that occur in the present or the future, whereas could is used for actions that occur in the past.

The structure of can and could is the following:

	Form	Examples
AFFIRMATIVE	Subject + can / could + infinitive	I can swim.
AFFIRMATIVE	Subject + can / could + infinitive	I could swim when I was 5.
NEGATIVE	Subject + can't / couldn't + infinitive	She can't come to the beach.
NEGATIVE	Subject + can't / couldn't + infinitive	We couldn't get the tickets.
INTERROGATIVE	Can / Could + subject + infinitive?	Can you stand on your head?
INTERROGATIVE	Can / Could + subject + infinitive?	Could you see the manager?

Can and could are followed by the infinitive without the particle to.

I can swim very fast / I can to swim very fast.

The negative forms of can and could are cannot and could not, but it’s much more common to use the contracted forms can’t and couldn’t.

Their main uses are:

To express physical or mental abilities.

He can't swim.

I could play the piano when I was 8.
To express the possibility to perform an action.

My car can go very fast.

I couldn't take him to the airport.
In addition, can and could are also used for requests and to ask for permission. In these cases, could is a bit more formal than can.

Can you pass me the salad?

Could you pass me the salad, please?

3. How do we use be able to?

Be able to has basically the same meaning as can and could, but unlike them, be able to can be used in any tense.

The structure of be able to is the following:

	Structure		Examples
PRESENT	+	Subject + am / is / are + able to + infinitive	I'm able to do it.
	-	Subject + am not / isn't / aren't + able to + infinitive	I'm not able to do it.
	?	Am / is / are + subject + able to + infinitive ?	Am I able to do it?
PAST	+	Subject + am not / isn't / aren't + able to + infinitive	You were able to arrive on time.
	-	Subject + wasn't / weren't + able to + infinitive	You weren't able to arrive on time.
	?	Was / Were + subject + able to + infinitive ?	Were you able to arrive on time?
PRESENT PERFECT	+	Subject + have / has + been + able to + infinitive	He has been able to open the tin.
	-	Subject + haven't / hasn't + been + able to + infinitive	He hasn't been able to open the tin.
	?	Have / Has + subject + been + able to + infinitive ?	Has he been able to open the tin?
FUTURE	+	Subject + will + be + able to + infinitive	We will be able to come to Paris.
	-	Subject + won't + be + able to + infinitive	We won't be able to come to Paris.
	?	Will + subject + be + able to + infinitive ?	Will we be able to come to Paris?

Be able to has the same uses as can and could.

For physical or mental abilities and to express the possibility to perform an action. Although it’s used in all tenses, in present simple can is more commonly used.

My sister is able to / can speak Japanese.

You aren't able to / can't fly.
There is a difference between could and was/were able to in affirmative sentences in the past when we talk about abilities. If we talk about ability in general, we can use both; whereas if we talk about an ability that took place in a specific situation, which involved certain effort, then only was/were able to is possible.

I could / was able to swim when I was young.

In this example we are talking about ability in general. Swimming is something that she just could do.

Finally, I was able to swim to the river bank.

In this example we are talking about a specific situation. He had to make an effort to get to the river bank.

However, there are exceptions. With the verbs see, hear, taste, feel, remember, understand, guess and believe, it’s preferable to use could instead of was/were able to.

Finally, they could understand his reasons.

They could hardly believe their eyes.
With other tenses, be able to will be used.

We have been able to fix your dress.

Will she be able to go on the trip?

When the modal verb of ability is preceded by a verb followed by an infinitive or it’s preceded by another modal verb, only be able to is possible.

I want to be able to sing like you.

I want to can sing like you.

You must be able to work hard.

You must can work hard.

Remember!

Can, could and be able to are all used to express physical or mental abilities and also to express possibility.

Form	Structure		Examples
CAN present	+	Subject + can + infinitive	I can play the piano.
	-	Subject + can't + infinitive	I can't drive.
	?	Can + subject + infinitive?	Can you speak French?
COULD past simple	+	Subject + could + infinitive	He could ride a horse when he was a teenager.
	-	Subject + couldn't + infinitive	She couldn't read when she was three.
	?	Could + subject + infinitive?	Could you dance salsa when you were young?
BE ABLE TO All tenses	+	Subject + be (correct tense) + able to + infinitive	I am able to fix the car.
	-	Subject + negative of to be (correct tense) + able to + infinitive	They weren't able to watch the movie.
	?	Be (correct tense) + subject + able to + infinitive?	Have they been able to help him?