We use this semi-modal to express possibility or the ability to do something, but unlike the pure modals, be able to has a full range of tenses and also needs to inflect to show agreement with its subject. For example:
He is able to offer you the best price possible.
We were able to get in to see the film.
They haven't been able to find the missing document.
So, you aren't able to help.
Notice that the negative is carried either by the be element or the auxiliary verb that is closest to the subject of the sentence. It can also be accompanied by any of the pure modals:
I will be able to see you after lunch.
They might not be able to put us up for the night.
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