"was able to" vs "could"

Question

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 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.

Answer 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.

Answer 3

1)According to 'A Practical English Grammar' by AJ Thomson et al, the one difference between could and be able to in the past affirmative is that could just implies that you had the general freedom or permission to do something, but may not have actually done or achieved it, where as be able to implies that you actually went ahead and did it.

2) Also, ** could ** does not have to be necessarily about a past event, it can also be used as a supposition of ability in the present. Was able to clarifies that it is indeed in the past.

Eg: He could save his parents from the sinking boat.

It is not clear whether this is past or present. If it is present not clear whether he actually did save them. He was able to save his parents

This emphasizes that he actually made an effort and succeeded in saving them.

So, I think in your example if the idea is to say that there was an expectation or permission that he could return to work, then it is OK. But the intended meaning is that he was well enough to do that, then we should use was able to.

Answer 4

"to be able to do" must be used when you need tenses that you can't express with can/could as this modal verb has only present and past tense. It is not possible to use can/could in future/conditional tense or the perfect tenses.

But as "to be able to do" is just a paraphrase for "can" you can always replace can/could by a form of to be able, theoretically. But of course, the modal verb is shorter and where can/could is possible you use normally can/could. But there is no difference of meaning between can/could and to be able.