Grammar notes: ability

Can and could

Can and could are modal verbs. They are sometimes called modal auxiliaries because they are generally used with another verb and help us to understand that verb.

Examples of this auxiliary use are:

In this section I'll only be looking at the 'ability' aspect of can and could.

General characteristics of can and could

They do not add -s in the 3rd person singular:

I can swim.
He can swim.
They could all drive before I could.
He could drive before I even had a bike.

To make a question you don't have to use an auxiliary verb, just invert the subject and the modal verb:

Can you swim?
Can he swim?
Could she read when she was three?

To make a negative sentence, add not or n't to the modal verb:

I can't swim (or I cannot swim. Cannot is one word)
Can't you swim?
We couldn't finish the exam in time.

Be able to

As I said above, can and could are modal verbs, and modal verbs don't follow the normal rules for verbs. For example, they don't have an infinitive or an -ing form.

For this reason can and could are impossible to use when you need to use the infinitive, the gerund or a continuous tense (though the continuous form would be impossible anyway!).

Being modal verbs also means they don't necessarily have a form that can be used for the past or the future, though in very general terms can is used to refer to the present and could is used to refer to the past.

When you can't use can or could, you need to use a form of be able to. This means exactly the same as can and could. Some examples:

I'm sorry, but I won't be able to come tomorrow.
Being able to speak another language is very useful.
You should be able to do this. It's easy!

Tense chart

To make things clearer, have a look at the chart below. It shows when you can use can, could and be able to.

Remember that be able to can always be used, but that English speakers use can and could whenever they can. Be able to sounds more formal and not English.

Notes about tenses

The chart above is only intended as a rough guide. As always with English verbs, a lot of the time the tense you use depends on what you're talking about.

For example, can can be used to refer to timetables or schedules in the future, just as the present simple is used normally:

I can't come tomorrow, but I can come at 3 o'clock next Thursday.

When talking about the past there's a difference between could and was/were able to: could is used in a more general sense while was/were able to is used to talk about more specific occasions:

I could run really fast when I was younger. But then I started smoking and my sister was able to beat me every time we had a race.

However, when you talk about the past in the negative couldn't and wasn't/weren't able to are completely interchangeable:

Sorry, I couldn't come yesterday.
Sorry, I wasn't able to come yesterday.

The conditional forms are also interchangeable:

I could go on holiday if I had more money.
I'd be able to go on holiday if I had more money.