"can" or "be able to"

[hela]

Dear teacher,

Which form should I use in the following sentence, both or just one ?
If I knew philosophy, I could / would be able to answer your question.

Are they perfectly identical ?

Many thanks,
Hela

[spoon]

Dear teacher,
Which form should I use in the following sentence, both or just one ?
If I knew philosophy, I could / would be able to answer your question.
Are they perfectly identical ?
Many thanks,
Hela

Actually, they are used interchangeably.
But there is something differences in usage between them.

a. If you mean that someone manages to do something in one particular situation, you have to use 'be able to'( not can).

The fire spread through the building very quickly, but everyone was able to escape.(not could)

b And 'be able to ' is used in cases(e.g.future, present perfect) where can/could is not grammatically possible.
One day scientists will be able to find a cure for cancer.(not will can find...)
I might be able to help you.(no I might can..)

c. 'be able to' is not usually followed by passive infinitives.
He can't be understood.(not He's not able to be understood.)

On the side of victory,
SpoonfulTM

[hela]

Thank you, Spoon