At the first sight, both the terms appear to be the same. For example.
I can walk for five km at a stretch.
This statement is talking about the present ability.
The same statement can be made alternatively using ‘be able to’
I am able to walk for five km at a stretch.
However, there are certain situations which necessitate the need for ‘be able to’
With Infinitive (to) – Whenever we have to use the infinitive to, we use be able to .For example.
To be able to get up early in the morning, I slept early at night.
Here with ‘to’ we should not use ‘can’.
After a Modal Verb- After a modal verb like might, we use only the second option.
He might be able to come on time, if it doesn’t rain in the evening.
In the above sentence, after one modal verb might, another modal verb ‘can’ is not used.
In the Present Perfect Sentence- In this tense also, ‘can’ is not used. For example.
He has been able to get over his difficulties after a long time.