Hi again, Max!
IMHO, "would be able" only refers to the notion of ability (in the contitional):
- If he studied English, he would be able to understand you.
- If my wrist weren't in a cast, I would be able to help you put these boxes away.
Whereas "could" may also refer to a logical possibility:
- Don't go there, it could be dangerous!
Plus "could" can also be past of "can")