Category: Adjectives & Adverbs
Able and unable are adjectives that refer to power, skill or possibility. Able is positive (ex. He is an able artist), while unable is negative (I am an unable dancer.). More frequently, able and unable are combined with be and an infinitive (to + dictionary form of a word) to make a structure that means the same as can or could.
Because can and could are modals and have no infinitive or participle forms, to be able to is often used in situations where can and could cannot be used.
In some cases, able and unable are used as adjectives to describe
Subject + be + able + noun
Ex. He is an able pianist.
More frequently, able and unable are used as part of a structure with the same meaning as can and could.
Subject + be able + infinitive (to + dictionary form of a verb)
Ex. I am able to come tomorrow.
Ex. It’s nice to be able to visit home.
Ex. I will be able to speak Spanish better after my home stay in Mexico.
Although be able has the same meaning as can/could, can/could is preferred when talking it means “know how to.” ex. I can ski is more natural than I am able to ski.
Can/could also usually preferred when followed by passive infinitives (be + past participle) ex. It can’t be understood. (more natural than It is not able to be understood.)
When talking about future possibilities based on currently abilities, can is used. However, when the ability or skill will be gained in the future, will be able to is used. ex. I am a good swimmer so I can swim in the race tomorrow. ex. I will be able to swim after taking the swim lessons.
See also Can/Could