We use can to say that someone has the ability or opportunity to do something. The negative of can is cannot (contraction: can't).
Can you swim?
He can play the guitar.
It's nice today. We can sit in the garden.
I can't open this bottle.
could for the italicized verbs if possible.
1. We had a good time yesterday. We went to the zoo. The children enjoyed themselves very much. They saw polar bears and elephants. (No substitution of 'could' is possible.) 2. When I lived in St. Louis, I went to the zoo whenever I wanted to,
but now I live in a small town...