Can, Could and Be able to



Can, Could and Be Able To Explanation

We use can  to say that something is possible or that someone has the ability to do something. The negative is can't (cannot). Let's see the following examples :
  • You can see the sea from our bedroom window.
  • Can you speak Spanish ?
  • I'm afraid I can't come your party tomorrow.

"Be Able To" Can Replace "Can"

Be able to is sometimes used to replace can. Let's see the following examples :
  • Are you able to speak Spanish?
  • I'm not able to see the sea from your bedroom window.

Present Perfect and Past Form of "Can" and "Be Able To"

Because can has only two forms : can (present) and could (past), and has no present perfect, you have to use be able to in the present perfect form. Also, can has no infinitive. Let's see the following examples :
  • I haven't been able to create any songs recently.
  • I will not be able to come to your birthday party tomorrow.

The past of can is could. We use could especially with these verbs:
  • see 
  • hear
  • smell
  • taste
  • feel
  • remember
  • understand

Let's see the following examples : 
  • When we went into the house, we could smell something burning.
  • She spoke in a low voice but I could understand what she was saying.
We also use could to say that someone had the general ability to do something. Let's see the following examples :
  • My grandfadher could speak five languages.
  • When Tom was 16, he could run 100 metres in 11 second.

"Be Able To" To Express Capability of Doing Something in Particular Situation

If you mean that someone managed to do something in one particular situation, you have to use was/were able to
(not could). Let's see the following examples :
  • The fire spread through the building very quickly but everyone was able (= managed ) to escape (not 'could escape  ')
  • They didn't want to come with us at first but in the end we were able (=managed)to persuade them. (not 'could persuade ')

Let's see the difference between could and was able to in this example:
  • John was an excellent tennis player . He could beat anybody.(= He had the ability to beat anybody.)
  • But once he had a difficult game against George. George  played very well but in the end John was able to beat him (= He managed to beat him in this particular game.)

The negative couldn't is possible in all situations. Let's see the following examples :
  • My grand father couldn't swim.
  • We tried hard but we couldn't persuade them to come with us.

Exercise 1

Complete the sentences below with can or be able to !

1. Tom .................. drive but he hasn't got a car.
2.  I can 't understand martin. I've never............. understand him .
3. I used to .................. stand on my head but I can't do it now.
4. Ask Ann about  your  problem. She should ............... help you.
5. I ... speak five languages.

Execise 2

Complete the sentences with could, was/were able to or couldn't !

1. He had hurt his leg, so he...walk very well.
2. She wasn't at home when I phoned but I ... contact her  at her office .
3. I looked very carefully and I ...  see a figure in the distance.
4. They didn't have any tomatoes in the first shop I went to, but  I ........................ get some in the next shop.
5. My grandmother loved music. She ........................... play the piano very well.

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