godan verb (-ku), intransitive verb
notice; realize; become aware of
Intransitive verb for becoming aware of something, often with に or と marking the noticed thing. Covers noticing, realizing, sensing, and suspecting.
彼は自分の間違いに気づいた。
He noticed his mistake.
外
Did you realize it's gotten dark outside?
godan verb (-ku), intransitive verb
Used when someone recovers from fainting or unconsciousness. Cross-referenced with 気がつく sense 3.
See also: 気がつく
彼は数分後に気づいた。
He came to after a few minutes.