"was able to" vs "could"

Solution 1:

In the first example, was able to is an alternative.

In the second example, could means that the speaker was in hearing distance of the phone, but implies that the speaker didn’t answer it, at least not immediately. If the speaker had said ‘I was able to hear the phone ringing’, the speech is more likely to continue with something like ‘. . . and so I went over and picked it up.’

In the third example, (1) is not necessarily wrong. It might occur in a sentence such as ‘The doctor said that after treatment he could return to work.’ (2) suggests that not only was he able to return to work, but that he did so.

Could and able to can be interchangeable, but the context will often decide which is chosen.

Solution 2:

A. There is one context in which could, with the sense of able to, is not usual. This is when referring to a single past occasion. For example:

? I was late but could get a good seat.

? We were tired but could reach the top of the mountain.

? How many points could you get in the test.

? I could get a bargain.

It is more usual to say was able to ..., succeeded in ..., or managed to ... in such contexts.

There are exceptions to this common avoidance of could when referring to single past occasions.

Exception 1: with verbs of the perception and mental activities:

I could hear a faint noise.

I could understand very little.

Exception 2: with negative or limiting adverbs:

I couldn't eat the fish.

I could only find three mistakes.

Exception 3: in some subordinate clauses:

I'm happy you could come.

The doctor said she could return to work.

B. In contexts where could refers to a general past ability (as opposed to a specific occasion as in A.), then could is common and interchangeable with was able to:

My deceased parrot could say words in three languages.

She could ride a bike when she three.

After the accident I could swim but not walk.