We use can to say that something is possible or allowed, or that somebody has the ability to do something.
We use can + base form (can do / can see, etc.):
We can see the ocean from our hotel window.
“I don’t have a pen.” “You can use mine.”
Can you speak any foreign languages?
I can come and help you tomorrow if you want.
The word “dream” can be a noun or a verb.
The negative is can’t (= cannot):
I’m afraid I can’t come to your party on Friday.
You can say that somebody is able to do something, but can is more common:
We are able to see the ocean from our hotel window.
But can has only two forms: can(present) and could(past). So sometimes it is necessary to use (be) able to.
I can’t sleep. – I haven’t been able to sleep recently.
Tom can come tomorrow. – Tom might be able to come tomorrow.
Maria can spanish, and English. – Applicants for the job must be able to speak two foreign languages.
Sometimes could is the past of can. We use could especially with:
We had a nice room in the hotel. We could see the ocean.
As soon as I walked into the room, I could smell gas.
She spoke in a very soft voice, so I couldn’t understand what she said.
We also use could to say that somebody had the general ability or permission to do something:
My grandfather could speak five languages.
We were totally free. We could do what we wanted. (= we were allowed to do)
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):
The fire spread through the building very quickly, but fortunately everybody was able to escape / managed to escape. (not could escape)
We didn’t know where David was, but we managed to find / were able to find him in the end. (not could find)
Jack was an excellent tennis player when he was younger. He could beat anybody. (= he had the general ability to beat anybody)
but Jack and Ted played tennis yesterday. Ted played very well, but Jack managed to / was able to beat him. (= he managed to beat him this time)
The negative couldn’t (could not) is possible in all situations:
My grandfather couldn’t swim.
We looked for David everywhere, but we couldn’t find him.
Ted played well, but he couldn’t beat Jack.