Let me start by saying what is written in grammar books on this issue and after that I will put my question. (Take heed that this usage of "could" ONLY refers to the past and ONLY to affirmative sentences)
1) "Could" shouldn't be used to address a single action/achievement in the past but it can be used to address a general ability in the past.
The examples are from M. Swan's grammar.
a) When I was younger, I could run 10 km in under 40 minutes. (CORRECT) b) I could run 10 km in under 1 hour yesterday. (WRONG)
It's all been clear so far. But here are a few answers of native speakers which I have got trying to elaborate.
Examples and opinions of native speakers on which is correct/incorrect. *1) First native speaker's examples* "I could visit you yesterday" - (incorrect) "I could do it yesterday" - (correct) "I could see you yesterday" - (correct)
My thoughts I don't quite understand why "I could visit" is incorrect while "I could do/see" is correct. Does it have to do something with single/multiple action?
"I could visit you (only ONE TIME) yesterday" if (only ONE TIME) then it's (incorrect) "I could do it (MANY TIMES) yesterday" if (MANY TIMES) then it's (correct) "I could see you (MANY TIMES) yesterday" if (MANY TIMES) then it's (correct)
2) 1) Second native speaker's examples
"I could visit you yesterday, but I didn't want to" - (correct) "I could kill everyone at work last week, but I couldn't have done it." - (correct)
I am confused as for why in this case "I could visit you yesterday" is considered acceptable. Does it mean that: "I could visit you yesterday" may have more than one meaning? Does it mean that "I could run 10 km yesterday" can also mean something and be correct?
Plus one more question. What does it mean? Does it refer to the future or the past
You could ask me before calling him. (WHEN? yesterday or in general?)