に気がつく【にきがつく】

に気がつく means "to notice" or "to realize": something you had not seen or understood comes to your attention. The thing you notice takes に. A noun goes straight before it; a whole clause in the plain form takes こと or の first.

The thing noticed never takes を, even though English makes it an object.

• × 間違いを気がついた → 間違いに気がついた

つく is a regular う-verb: きがついた気がついた, きがつかない気がつかない, きがつきます気がつきます.

気がついた or 気がついている

気がついた is the moment of noticing. 気がついている is the state after it: you have noticed and are now aware. English says "I realize" or "I know" for the second, so don't read it as "I am noticing".

気づく

きづく気づく is the same expression contracted into one verb. It takes に in the same way and means the same, and the two swap freely in speech and writing.

気をつける is a different expression

きをつける気をつける with を means "to be careful". The two differ by one particle.

Usage notes

These go beyond noticing a thing or a fact.

よく気がつく

With よく and no object, it describes someone attentive, who notices what other people need.

Coming round

Without an object, 気がつく also means to come round after fainting or sleeping. きがつく気がつくと at the start of a sentence means "the next thing I knew".