Talking about ability, possibility and obligation

I can swim, but I can't play tennis.

I can play basketball, but I can't do gymnastics.

to talk about something we are/ aren't able to do at this moment.

I couldn't swim when I was five.

I could ride a bike when I was 9.

To talk abou something we were/ weren't able to do in the past.

+ She could do it.

- I couldn't stand up.

A friend is coming mountain climbing with me for the first time and I want to tell him what he can expect to happen.

Can or can't to talk about possibility.

+ It can be dangerous.

- You can't play tennis today; you haven't got your racket.

To talk about something that is/ isn't necessary.

To talk about something that was necessary.