Could is historically a past tense form of 'can' (cunne). It refers to "possibility or capability".
Like all modal verbs it can also indicate a degree of probability:
- Is that Maria?
- Could be. (= maybe)
- Was that Mario at the door?
- No, that couldn't have been Mario, I'm videoconferencing with him, he's walking across the bridge at the moment.
Could is a tricky word. It is used for both the negative past and the conditional or hypothetical future.
One of the most frequent mistakes I read in emails is:
I couldn't come tomorrow.(incorrect)
- I can come tomorrow. (correct)
Could can be used to speak about a hypothetical future, but couldn't cannot be used to speak about the future:
- I could come to your office once the lockdown ends.
I couldn't come to your office once the lockdown ends.
- I could send it to you by email. (if you would like)
I couldn't send it to you by email.<-- necessarily interpreted as the past tense (je ne pouvais pas te l'envoyer par mél.)
Couldn't can also be used for the past tense. However, could is not generally used for the past tense.
- I couldn't come to your office in April 2020 because of the lockdown.
I could see my clients yesterday.(use "was able to" instead)
There are many exceptions to this overly simple rule! (concessive or contrastive clauses, when used with focusing adverbs like only, even, or hardly...)
- I could see my clients yesterday, but I couldn't hear them.
- Although I could see my Skype clients this morning, I could hardly hear them.
- I could only hear my clients properly using Zoom.
Both could and couldn't can be used in the past when followed by a perfect infinitive (e.g. have been):
- The pandemic could have been worse (if there hadn't been a lockdown).
- The timing of this shutdown couldn't have been worse for some businesses.
- The timing of this shutdown couldn't have been better for workers who needed a vacation.
In the past (and the conditional or "future of the past") "be able to" may be used.
- was(n't)/were(n't) able to
- I was able to see my clients yesterday.
- would(n't) be able to