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. (correct)

Hypothetical Future

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.)

Past

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.

Nevertheless there are many exceptions to this rule!

• Although I could see my Skype clients this morning, I couldn't hear them.
• I could see my clients yesterday, but I couldn't hear them.
• I could only see my clients using Zoom, Whatsapp or Skype in April 2020.
• The pandemic could have been worse (if there hadn't been a lockdown).

Periphrastics

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