Why do some things sound right and other things sound wrong? DUH! An easy answer would be that some things ARE grammatically right and others wrong, but I'm trying to steer clear of "grammatically incorrect" and prefer "linguistic variation". Some things just sound right or wrong
can, could, be able to

The book's grammatical explanation in the difference in use of could and be able to, is that we use could OR able to for general ability; e.g. He could / was able to play piano when he was 6 years old. However, according to the book, only able to is used to talk about managing a specific difficult situation, e.g. The door was stuck fast but he was able to open it using his great strength. Then in the exercise there are two sentences of a "specific difficult situation":

a. They ____________ buy the flat because it was too expensive.
b. There was a lot of traffic but we _____________ get there on time.
according to the explanation we should use be able to.

Try it. Now for me the first sounds much better with couldn't and the second sounds much better with were able to. Why?

(ref. English File, Intermediate Student's book. Oxford University Press. Oxford, 1999)