Although we look at be able to here, it is not a modal verb. It is simply the verb be plus an adjective (able) followed by the infinitive. We look at be able to here because we sometimes use it instead ofcan and could.
We use be able to:
- to talk about ability
Structure of Be able to
The structure of be able to is:
subject + be + able + infinitive
|-||She||is not||able||to drive.|
Notice that be able to is possible in all tenses, for example:
- I was able to drive...
- I will be able to drive...
- I have been able to drive...
Notice too that be able to has an infinitive form:
- I would like to be able to speak Chinese.
Use of Be able to
Be able to is not a modal auxiliary verb. We include it here for convenience, because it is often used like "can" and "could", which are modal auxiliary verbs.
be able to: ability
We use be able to to express ability. "Able" is an adjective meaning: having the power, skill or means to do something. If we say "I am able to swim", it is like saying "I can swim". We sometimes use "be able to" instead of "can" or "could" for ability. "Be able to" is possible in all tenses - but "can" is possible only in the present and "could" is possible only in the past for ability. In addition, "can" and "could" have no infinitive form. So we use "be able to" when we want to use other tenses or the infinitive. Look at these examples:
- I have been able to swim since I was five. (present perfect)
- You will be able to speak perfect English very soon. (future simple)
- I would like to be able to fly an airplane. (infinitive)