un‧a‧ble W2 / ʌneɪb ə l / 形容词 [名词前不用]

not able to do something → inability

unable to do something

Lucy was unable to find out what had happened.

Unable to sleep, I got up and made myself a drink.

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In everyday English, people usually say that they can’t/couldn’t do something rather than are/were unable to do something :

She couldn’t find out what had happened.