Material unit 2

Expressing ability

1. Present

We can also say 'I am able to run very fast' but we prefer 'can' because it is shorter:

I can run very fast.

2. Past affirmative

We use 'could' to speak in general:

I could run very fast, too.

We use 'was able to' to refer to a particular occasion. We are not speaking in general:

I could run very fast. That's why I was able to escape from the policeman.

We use 'could have' for something which was possible, but we didn't do. It is connected to the third conditional.

3. Past negative

There is no difference between 'couldn't' and 'wasn't able to'. 

I couldn't run fast.
I wasn't able to run fast.

4. Other ways of expressing ability

In any tense, we can emphasise that the activity was difficult by using 'manage to':

I managed to open the bottle.

In any tense, we can emphasise that we were successful by using 'succeed in + gerund':

I succeeded in winning the cup.