All the grammar tips say that "could" can't be used when it's meaning a single past event. They say all the things like hypothesis mood or general ability or whatnot.
But, then again, at the end of their explanation, they would usually put an exception of this rule for certain verbs or adverbs. Why is it so? If you say like
I could hear his voice yesterday night.
,then would the verb 'hear' make the conditional mood wear off? If so, how come?
English is not a language of strict rules, it is a language of exceptions. Sometimes these occur because words that now seem to have similar forms have very different histories. Sometimes that is just how usage has gone. There are very few absolute rules in English with no exceptions.
I could hear his voice yesterday night.
means
I was able to hear his vice yesterday night.
"Could" can be used for other single past events where the meaning is "was able to".
- I could lift my leg yesterday, but now it's too sore.
- I could take Main Street last week, but now it's torn up.
- I could run a 5-minute mile last year, but I'm out of shape.
- I could see through this window yesterday, before the dust storm.
Any grammar text that claims that "could" can't be used when it's meaning a single past event is simply missing a category of use.
