Je gebruikt could om aan te geven of vragen dat iets 'zou kunnen' of 'zou mogen'
I could help you with that if you like.
Could I have a cup of tea please?
Je gebruikt could om aan te geven dat iets 'kon'
She could swim very well when she was younger
Je gebruikt could not om aan te geven dat iets 'niet kon'
He could not talk until he was five.
Could is beleefder dan can bij het vragen om hulp / toestemming.