can, could, be able to and be allowed to

Can, could and be able to: talking about ability
avoid use be able to

• when we talk about something that is happening as we speak
• before passive
• when the meaning is "know how to"

if we talk about single achievement, rather than a general ability in the past, we usually use be able to rather than could.

Can and could: talking about possibility
theoretical possibility of something happening we use could, not can

We use can to indicate that there is a very real possibility of a future event happening. Using could suggests that something is less likely or that there is some doubt about it.

Could and be allowed to: talking about permission

The future seen from the past

1. In order to express this idea wee can use the past tenses of the verb forms we would normally use to talk about the future.

Ex:

I'm not going to say anything about the exams today, because I don't have time.

I wasn't going to say anything about the exams, but the students asked me to. 

2. for things that were expected, but didn't happen

Ex:
He was to find out years later that the car he had bought was stolen.
I was supposed to help, but I was ill.

Future in another way

In phrases to refer to actions or event in future with a meaning be about to + infinitive
be on the verge of ...
brink of ...
point of...