Depends on how it is used.
can = able to
We can sign up for class tomorrow only
can = allowed to
You can paint your room purple if you want.
can = have the right to
You can vote at age 18.
:lol:
Also their certain verb tenses that "can" can't handle, so we use "be able to":
I haven't been able to do anything so far.
You'll be able to do it when you're older.
Being able to speak English is becoming more and more important.
I used to be able to touch my toes without bending my knees.
Lou