Grammar: Can, could and (be) able to

Read the rules:

Can

We use can to say that something is possible or allowed, or that somebody has the ability to do something. We use can + infinitive (can do / can see etc.):

1. We can see the lake from our bedroom window.
2. "I haven’t got a pen." "You can use mine."
3. Can you speak any foreign languages?
4. I can come and see you tomorrow if you like.
5. The word "play" can be a noun or a verb.

The negative is can’t (= cannot): I’m afraid I can’t come to the party on Friday.

Could

Sometimes could is the past of can. We use could especially with: see, hear, smell, taste, feel, remember, understand

1.  We had a lovely room in the hotel. We could see the lake.
2. As soon as I walked into the room, I could smell gas.
3. I was sitting at the back of the theatre and couldn't hear very well.

We also use could to say that somebody had the general ability or permission to do something:

1. My grandfather could speak five languages.
2.  We were totally free. We could do what we wanted. (= we were allowed to do)

Can и Is able to

You can say that somebody is able to do something, but can is more usual: We are able to see the lake from our bedroom window.

But can has only two forms: can (present) and could (past). So sometimes it is necessary to use (be) able to. Compare:

1. I can’t sleep. - I haven’t been able to sleep recently.
2. Tom can come tomorrow. - Tom might be able to come tomorrow.
3. Maria can speak French, Spanish and English. - Applicants for the job must be able to speak two foreign languages.

Could and was able to

We use could for general ability. But if you want to say that somebody did something in a specific situation, use was/were able to or managed to (not could):

1.  The fire spread through the building very quickly, but fortunately everybody was able to escape / managed to escape. (not could escape)
2.  We didn't know where David was, but we managed to find / were able to find him in the end. (not could find)

Compare:

1. Mike was an excellent tennis player when he was younger. He could beat anybody. (= he had the general ability to beat anybody)
2. Mike and Pete played tennis yesterday. Pete played very well, but Mike managed to beat him. (= he managed to beat him in this particular game)

The negative couldn't (could not) is possible in all situations:

1. My grandfather couldn't swim.
2. We looked for David everywhere, but we couldn't find him.
3. Pete played well, but he couldn't beat Mike.