This document discusses the uses of "can", "can't", "cannot", "be able to", and "could" when referring to ability or permission. It provides examples of how each is used in sentences. Specifically, it notes that "can" is used to describe present ability or lack thereof, as well as future meaning and with verbs of perception. Meanwhile, "be able to" has the same meaning as "can" but is used instead when "can" is unsuitable, or for non-present tenses. Finally, "could" refers to past ability that was demonstrated, while "be able to" or "managed to" emphasize success in accomplishing something difficult on a specific occasion in the past.

1. Can – Could – Be able to

2.  Bertie says, “I can’t speak!” Why? Because he
wasn’t able to speak without pauses or without
repeating the initial letters of words. He was a
stammerer!
 The swimming instructor says: “I have not had
anyone who hasn't been able to do it within
about five minutes.”
Examples

3. Can, can’t, cannot
We use can / can't
• to describe ability or lack of it.
My son can speak English fluently, but he can’t speak French.
• to describe what is allowed or not allowed.
You can visit Buckingham Palace but you can’t talk to the King.
• with a future meaning.
Sorry, but I can't come to the party next week.
• with verbs of perception: hear, see, taste, feel, smell.
What can you see?
I can smell gas!

4. Be able to
This has the same meaning as can.
We use be able to
• instead of can.
The King wasn’t able to speak easily, he stammered.
• when can is unsuitable. Because can has only present
and past forms, we use be able to for other tenses, and
infinitive form.
I haven't been able to finish my project.
We hope to be able to visit you next month.

5. Could vs Be able to
In an affirmative past simple sentence, we use “be able to”
or “managed to” if we want to refer to something that
someone succeeded in doing, something difficult on a
specific occasion.
Although I was tired, I was able to finish my work.