Can, Could And To Be Able To Exercise.pdf _VERIFIED_
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can and could are modal auxiliary verbs. be able to is NOT an auxiliary verb (it uses the verb be as a main verb). We include be able to here for convenience.
We use I (positive) and couldn't (negative) for general ability in the past. But when we talk about one special occasion in the past, we use be able to (positive) and couldn't (negative). Look at these examples:
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 of can and