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.