[be] able to vs can
We use able especially in the structure be able + infinitive. This often has the same meaning as can.
There is a negative form unable.
Some people are able to / can walk on their hands.
I am unable to I can't understand what she wants.
Can is preferred in the sense of 'know how to', and in expressions like can see,
can hear etc
Can you knit?
(More natural than Are you able to knit?)
I can see a ship.
(More natural than I am able to see a ship.)
Be able is used in cases (e.g. future, present perfect) where can/could is not grammatically possible because it has no infinitive or participles
One day scientists will be able to find a cure for cancer.
(NOT ••• will can find ...)
What have you been able to find out?
(NOT What have you could ... ?)
I might be able to help you.
(NOT I might Can ..•)
Able is not often followed by passive infinitives.
He can't be understood.
(More natural than He's not able to be understood.)
Note
about and on
Compare:
- a book for children about Africa and its peoples
a textbook on African history
- a conversation about money
a lecture on economics
We use about to talk about ordinary, more general kinds of communication.
On suggests that a book, talk etc is more serious, suitable for specialists.
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