Ability: can, can't, could, couldn't

1. We form sentences with can like this:

can+ infinitive

I can ski.

Positive

I/He/She/It/We/You/They can ski

Negative Full Short
Form Form

I/He/She/It/We/You/They cannot ski can't ski.

Questions

Can I/He/She/It/We/You/They (etc) ski?

In spoken English cannot is possible, but we normally use can't:

He can't swim

2.- We use can and can't to talk about things we are able to do generally.

She can speak Japanese. (= She is able to speak Japanese)
He can't ski. (= He isn't able to ski.)

We also use can and can't to talk about things we are able to do at the moment:
I can see the moon. (= I am able yo see it now).

3.- We form sentences with could like this:

could + infinitive

I could swim

Positive

I/He/She/It/We/You/They. could ski.

Negative Full Short

Form Form

I/He/She/It/We/You/They could not ski. couldn't ski.

We use could to talk about things we were able to do generally in the past:

I could run 100 metres in 12 seconds when i was young

Susan could read when she was three years old.

4.- We do not normally use could for something that happened on an particular occasion in the past. We use was able to or managed to:

The boat was in difficulties, but in the end it managed to reach the port. (Or... it was able to reach...; Not... it could reach...)

5.- When we talk about person's ability to do something in the future, we use will be able to.

The baby will be able to talk soon.