В пособии рассматриваются основные моменты английской грамматики: части речи имя существительное, имя прилагательное и наречие; видо-временные формы глагола; модальные глаголы; неличные формы глагола

be able to is also possible but less usual.
But in some patterns we always use be able to, not can:
To-infinitive: It’s nice to be able to go to the opera.
Present perfect: Since his accident he hasn’t been able to leave the house.
To talk about future ability, will be able to is normally used.
Our baby will be able to walk in a few weeks.
But we normally use can (not will be able to) to suggest a possible future
action or to make present decisions about future ability. Could is also
possible here, but is more unsure.
We’re busy today, but we can repair your car tomorrow.
Let’s have lunch together. We can go to that new restaurant.
‘What shall we do this evening?’ - ‘We could go to the cinema’.


B. Past
Could is used for ‘general ability’, to say that you could do something any
time you wanted to. (Was/were able to is also possible.)
She could sing like an angel when she was a kid.
My father could speak ten languages.
Could is not used to talk about particular ability (to do something on one
occasion) in the past. Instead, we use was able to, managed to, or succeeded
in.
Luckily I had my camera with me, so I was able to play in the match.
The driver managed to stop just before he hit the wall.
After six hours’ climbing, we succeeded in reaching the top.
But in the negative couldn’t is used for both general and particular ability.
When I was younger I couldn’t decide what I wanted to do.
(general ability)
Simon was so tired that he couldn’t open the door. (particular ability)
C. Could with the Perfect Infinitive
1. This form is used for past ability when the action was not performed. We
had the ability to do something but we didn’t try to do it.
I could have lent you the money. Why didn’t you ask me?
I was so angry! I could have killed her.
It is also used when we don’t know whether it was performed or not:
Someone could have taken the keys from your bag.
2. Could + perfect infinitive can also express irritation at or reproach to the
non-performance of an action:
You could have told me.
3. We use couldn’t have to say that it is impossible that something
happened:
Paul couldn’t have taken a bus yesterday. There aren’t any buses
on Sundays.
I coudn’t have enjoyed myself more - it was perfect.


D. Could with remember etc.
With some verbs can is often left out without changing the meaning very
much. This happens with remember, speak, understand and play.
I (can) remember London during the war.
She can speak Greek/She speaks Greek.
I can’t/don’t understand.
Do/can you play the piano?