A grammar book says that in the following sentences we must not use "could" instead of "was/were able to".
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The fire spread through the building very quickly, but everyone was able to escape.
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They didn't want to come with us at first, but finally we were able to persuade them
However, when I searched the English language corpus COCA by "finally could", I found some counter-examples. Here are two examples from COCA.
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I told her to get in line. Did it make you feel better? It did.
I finally could say something. -
Then, when it ended and I finally could get my family back,
it came at a price, like suddenly being blind.
Are these examples grammatically incorrect?
Edit(Jun. 5, 2014)
I posted the same question here.
Best Answer
I believe that you could replace every were able to in your examples with could, but it might not mean what you think it means, and it could sound strange or misleading enough that some grammar books make it a rule that you must not use it. For example,
However, I think this is a little too simplified, and it could cause confusion, even for advanced learners. In my opinion, it is easier to think that
And because of that, was/were able to is preferred when we are talking about the successful completion of a specific attempt. Though I believe that this might not be a hard-and-fast rule. The was/were able to always suggests that it's very likely that the attempt was successfully made. The managed to and succeeded in also suggests so, and the achievement is even more definite.
Let's consider the examples:
Now the examples from COCA,
This should answer your question "Are these examples grammatically incorrect?".
Of course not. They are grammatically correct.