Could

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. (incorrect)

Now, could can be used to speak about a hypothetical future, but, strangely, couldn't cannot be used for a negative hypothetical 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.)

Could can also be used for a hypothetical future. However, strangely, couldn't cannot be used for a negative hypothetical 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 (except with the perfect infinitive, or with focusing adverbs like "only" or "even")

• I couldn't come to your office in April 2020 because of the lockdown.
• I could only see my clients using Zoom, Whatsapp or Skype in April 2020.
• I could see my clients yesterday.
• The pandemic could have been worse (if there hadn't been a lockdown).

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
• would(n't) be able to