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I'm a bit perplexed, if not lost.

Given "could" can't be used to describe a one-time ability and should be replaced by "was able to", such as in the following sentence:

The burglar was able to (not could) get into the house through the window.

Then what makes the next sentence correct?

I could see the look of disgust on her face.

I know it's fine, but according to the rule, shouldn't it be replaced by "was able to", since I was able to see the look on her face just on this one occasion?

And also, does the usage of time-related clauses, such as "yesterday", make the "was able to" form the only acceptable one?

Because I've become fluent in German, yesterday I could/was able to come to an agreement with our foreign client.

And Another example:

Once the riots in the prison started, I could/was able to escape, but decided to stay to avoid possible consequences.

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  • "should be replaced"? That's not a rule. "The burglar could get into the house..." and "I could see the look... " are both perfectly well-written sentences. – John Feltz Nov 23 '16 at 12:07
  • So what's the differece between the two forms in my burglar example? – Bebop B. Nov 23 '16 at 12:14
  • There's no difference; they are equivalent sentences. – John Feltz Nov 23 '16 at 12:16
  • There is a subtle difference which isn't worth-mentioning. Was able to/could are almost interchangeable in many cases. – yubraj Nov 23 '16 at 13:21
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The second sentence below, with could smash is not valid paraphrase of the first, with was able to smash.

The burglar was able to smash the window.

The burglar could smash the window.ungrammatical in context

But here, the substitution of could for was able is not discordant to my ear:

The toddler was able to chew solid food.

The toddler could chew solid food.

The sentence about the burglar is understood to be an explanation for a break-in. The window was smashed. It happened. Being able to smash a particular window is not the same thing as being able to chew now that your molars are in. The ability to chew solid food is not a momentary, discrete ability as was the smashing of the window. In the same way, seeing is not a momentary discrete action but something that continues or persists.

I was able to see the mountains in the distance.

I could see the mountains in the distance.

And could can be substituted for was able when referring to the persistent or to the ongoing, to actions or events which are not over and done with the moment they take place.

We can easily introduce discord by choosing a verb that refers to an action that takes place all at once, catch:

Rounding the corner, I was able to catch a glimpse of the mountains in the distance.

Rounding the corner, I could catch a glimpse of the mountains in the distance.marginal

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  • Thank you, that sheds some light on my problem. But still, I'm not sure which form I should choose in my last example, the one with the prison. Is my escape considered a discrete action, or is it ok to stick to "could"? – Bebop B. Nov 23 '16 at 14:48
  • I think you're painting yourself into a corner there by avoiding the optimal choice of tense. You might say: "I could have escaped once the riots in prison had started, but decided...." or "Now that the riots had started, I could escape, but ..." Was able to implies the escape took place (as was typically refers to a past actual not a past hypothetical) and then you reverse the meaning with "but". – Tᴚoɯɐuo Nov 23 '16 at 14:52
  • And the German example? If I get it right, the "was able to" is more correct? – Bebop B. Nov 23 '16 at 15:00
  • With the prison break, you're talking about a past hypothetical ("could have escaped"); with the German example, it seems you're talking about something which did indeed take place, a past actual: "Because I've become fluent in German I was able to strike a deal with our foreign client yesterday". "Strike a deal" is like "catch a glimpse" or "come to an agreement" in terms of its being a discrete action in time. – Tᴚoɯɐuo Nov 23 '16 at 15:04

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