grammar - "was able to" vs "could"

According to my grammar book, here are some usages of was able to and could

• could can be used to refer in general that someone has a skill.
  e.g. At that time I could still read without spectacles.

  
  
  

  In that sentence, could could be replaced by was able to?
  Would there be any difference in meaning ?




• When could is used with words such as hear, see, understand, etc., it means that someone can do something specific.
  e.g. I could hear the phone ringing.

  
  
  

  In that sentence, could could be replaced by was able to?
  Would there be any difference in meaning ?




• When could is not used with *hear, see, understand,*etc., it can't indicate that someone has the ability to do something specific.
  e.g.:

  
  
  
  
  
  
  1. After treatment he could return to work. ( wrong )
  
  
  2. After treatment he was able to return to work. ( correct )
  
  
  
  
  

  Why is the first sentence wrong?
  What if it comes to someone gets approval to do something? Which form is better in that case?

Can was/were able to refer both to having the skill or ability to do something?

When are could and was/were able to interchangeable?

I would like to know how native speakers use these two terms in daily life.

Answer

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.