"could" vs "were able to"

[optimistic pessimist]

Dear all,

I understand you normally use "were able to", not "could", when you refer to a discrete past event.

e.g. NG : Against all odds they could finally reach the top of the mountain.

OK : Against all odds they were finally able to reach the top of the mountain.

If so, is the example below fine?

" The trouble was, " she went on before I could say anything, " I misunderstood what you told me. Of course, now I know we wouldn't be here if I'd done what you said.... " At that point Kiya covered her eyes with one hand. I figured she was getting ready to admit that she'd released the nanomachines herself. But before she could say a single word the ground beneath us shook violently and maelstroms of silver bubbles began swirling silently around us, dragging us down to the bottom. Within seconds the ...

I found this passage in COCA.

OP

[billmcd]

Dear all,

I understand you normally use "were able to", not "could", when you refer to a discrete past event.

e.g. NG : Against all odds they could finally reach the top of the mountain.


OK : Against all odds they were finally able to reach the top of the mountain.

If so, is the example below fine?

" The trouble was, " she went on before I could say anything, " I misunderstood what you told me. Of course, now I know we wouldn't be here if I'd done what you said.... " At that point Kiya covered her eyes with one hand. I figured she was getting ready to admit that she'd released the nanomachines herself. But before she could say a single word the ground beneath us shook violently and maelstroms of silver bubbles began swirling silently around us, dragging us down to the bottom. Within seconds the ...

I found this passage in COCA.

OP

Yes but so is "before I was able to".

[Raymott]

...

e.g. NG : Against all odds they could finally reach the top of the mountain.

I'm not sure what NG means. But I guess you're saying it's not correct -- which is true.

" The trouble was, " she went on before I could say anything, ...

But this didn't happen, so it's not referring to something that she was able to do. It refers to something she was not able to do.
"Whatever they did, they could not reach the top of the mountain." This is correct.

[BobSmith]

NotGood : Against all odds they could finally reach the top of the mountain.

[not a teacher]

I think there is a case were "could" could be used, but I'm not sure why (other than "could" here connotes "enabled"):

With the helicopter repaired, they could finally reach the top of the mountain.

Is this ok? Perhaps a teacher could provide edification.