un ‧ a ‧ ble W2 /ʌnˈeɪb ə l/ BrE AmE adjective [not before noun]
[ Word Family: noun : ↑ ability ≠ ↑ inability , ↑ disability ; adjective : ↑ able ≠ ↑ unable , ↑ disabled ; verb : ↑ enable ≠ ↑ disable ; adverb : ↑ ably ]
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 :
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She couldn't find out what had happened.