en:grammar:modals:to_be_able

Birds are able to fly.

To be able to is a structure which is sometimes used as an alternative to the modal auxilary verb can.

Affirmative: subject + 'be able to' + bare infinitive

I am able to swim.

Negative: subject + 'be' + 'not' + 'able to' + bare infinitive

I am not able to swim.

Interrogative: 'be'+ subject + 'able to' + bare infinitive

Are you able to swim?

Use

Examples

As an alternative to can for ability or possibility in the present tense

I'm usually able to take 3 weeks holiday in August.

We're able to see the coast of France on a clear day.

When could cannot be used in the past tense. [See note 3. below]

I was finally able to have a meeting with them last week.

When can is not possible - as an infinitive or gerund, when we talk about the future and with modal auxiliary verbs

It's important to be able to swim.

I must be able to speak fluent English before I go to London.

I can't finish the report today but I'll be able to do it tomorrow.

Note that

1. In the present simple and when there is a choice, can is usually preferred.

2. We avoid 'to be able to' when something is happening at the moment:

"Look at me - I can dance like Shakira!"

3. There is a difference between using could and 'was able to' - see could for an explanation.

After taking lessons, Camille is able to swim underwater.

I had a problem with my car yesterday but, fortunately, I was able to get a taxi to the airport.

"It's useful in my job to be able to write in English."

"Being able to speak English is more important in my department."

"Can you come to my office later to discuss this?"

"I might be able to. I'll call you later to confirm."

Oh, no not again! I hope I'll be able to fix it before tomorrow!

To be able to