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I wonder whether we can use couldn't in this sentence, and if no, why:

She misses her family living abroad. She __ visit them for years, but they just got an e-mail account, so now they can keep in touch daily.

A test I've been solving says one should use hasn't been able to, not couldn't, but I'm not sure why.

One textbook (Downing & Locke) says that we use 'could' for extended actions in the past and 'was able to' for perfective actions, but that in negative phrases this distinction is not obligatory:

He was able to escape. (OK)

*He could escape. (Error)

But:

He wasn't able to escape.

He couldn't escape.

Is it that the words "for years" together with the "but-clause" containing the word "just" stress the "perfectivity" and force us to use the construction (to be able to) better suited for perfective actions?

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Note that there are more than two possibilities here. You could use "couldn't", "wasn't able to", or "hasn't been able to". The reason "hasn't been able to" is better than either of the other two is that she still can't visit them. The present perfect is generally the correct verb tense to use when you have a condition that extends from the past into the present, and "hasn't been able to" is present perfect, while both "couldn't" and "wasn't able to" are simple past.

In both the sentences:

She couldn't visit them for years, but …
She wasn't able to visit them for years, but …

the word but sets up the expectation that the first clause is no longer true. However, in the sentence

She hasn't been able to visit them for years, but …

this doesn't happen; because you used the present perfect, the listener knows that the first clause is still true, and so is not surprised when the second clause fails to contradict the first.

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  • I really haven't thought about that! Thank you! I wonder if wasn't able to would be OK or would it, just like couldn't, also erroneously imply that she is now able to visit them. I guess hasn't been able to is the only 'smooth' option, ie without such implication.
    – CowperKettle
    Commented Mar 2, 2014 at 16:42
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    The expression "wasn't able to" works exactly like "couldn't" here (which is what your textbook says happens in negative phrases), so neither of them is the best choice. I can't decide whether I think they're actually grammatically wrong or not, but you can make a good argument that they are wrong, so this test question is reasonable, although quite tricky.
    – Peter Shor
    Commented Mar 2, 2014 at 16:54
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"hasn't been able to" is present perfect, which emphasises that something happened up until something else in the present:

"She hasn't been able to visit them for years, but they just got an e-mail account",

"Just got" is an action in the present that is preceded by the past action of not being able to visit them.

"could" indicates something that was completed before something else:

She couldn't visit them for years, but now they can keep in touch daily.

This "could not" is over, replaced by a can now (grammatically speaking; as Peter Shor points out, she still can't actually visit them).

Note that it is the "can now" that is emphasised, not the "just got". "Could" would be better in a slightly different sentence:

She couldn't visit them for years, but they can now keep in touch daily (because they now have an e-mail account).

vs

She hasn't been able to visit them for years, but they just got an e-mail account (so now they can keep in touch daily)

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