Tham khảo ngữ pháp

Present and past modals of ability

Meaning and use

We use can or be able to for saying that somebody or something has the ability to do something. Can and be able to mean the same, but we often use be able to when something is surprising or unusual.

Camels can carry up to 600 pounds on their backs.
They are able to close their noses to keep out the sand.

To talk about ability in the past, we use could or be able to.

20 years ago, the Kalahari bushmen could make fire without matches.
They were able to survive without fresh water for three weeks.

For a single event in the past, we use be able to (not could)in positive statements.

Our guide was able to show us the caves where the bushmen lived.
NOT: Our guide couldshow us the caves where the bushmen lived.

However, in negative statements and questions about single events, we can use could or be able to.

Could you talk to the bushmen? Were you able to talk to the bushmen?
We couldn't talk to them. We weren't able to talk to them.

If something is very difficult to do or is very successful, we often use the verb manage to instead of a modal verb.

The bushmen managed to live happily in their natural environment for at least 20,000 years. 

Form

After can/can't and could/couldn't we use the infinitive without to. We also use the same form for all persons: I, you, he, she, it, we, they. After be able to we use the infinitive without to as well, but the form of be changes: am/are/is able to for the present and was/were able to for the past.

Positive

In Yakutia, the people can speak both Yakut and Russian.
They are able to survivein temperatures of -70.

Negative

They can’t grow vegetables in winter, so they eat a lot of meat and fish.
Most people aren’t able to travel in winter because of the freezing weather.

Questions

In questions with cancould and be able to, we change the word order. We don’t use Do/DoesCan, could or the verb be come before the subject or subject pronoun.

How can the people survive in temperatures of -70?
Why aren’t they able to travel in winter?
Could they leave the area by April?

Take note: 'be able to' with infinitives and present perfect

Can has no infinitive form or present perfect form. So in some sentences, we have to use be able to.

I’d like to be able to speak Russian. (infinitive)
We haven’t been able to contact them by phone yet. (present perfect)

Take note: form of 'manage to'

Manage to is a regular verb in the positive form. But in the negative form we say can’t manage to in the present and couldn’t or didn’t manage to in the past. We also use did for questions.

can’t manage to eat all that!
They couldn’t/didn’t manage to finish the race.
Did you really manage to swim across the Channel?

Take note: sense verbs and verbs of thinking

We usually use can and not be able to with sense verbs and verbs of thinking such as see, hear, smell, believe, remember.

can see the Lena River from here.
can’t believe that it gets so cold there.
The bushmen could remember their way around the desert from day to day.