adjectives
(usually followed by `to') not having the necessary means or skill or know-how
"unable to get to town without a car"; "unable to obtain funds"
is an attribute of
the quality of being able to perform; a quality that permits or facilitates achievement or accomplishment
(usually followed by `to') lacking necessary physical or mental ability
"dyslexics are unable to learn to read adequately"; "the sun was unable to melt enough snow"
is similar to
(followed by `of') lacking capacity or ability
"incapable of carrying a tune"; "he is incapable of understanding the matter"; "incapable of doing the work"
lacking in power or forcefulness
"an ineffectual ruler"; "like an unable phoenix in hot ashes"