What it means
に気がつく means to notice something or become aware of something. The noun marked by に is the thing that enters the speaker’s awareness. It can describe noticing a mistake, a sound, a change, or another fact. The familiar form is commonly used in plain statements, while 気がつきます is the polite counterpart.
Usage note
に気がつく uses に for the object of awareness, not を. Do not confuse it with 気をつける, which means to be careful or to pay attention.
How to use it
Nounに気がつきます
私はすぐに自分の間違いに気がつきます。
I quickly notice my own mistake.
Clause (plain form)ことに気がつきます
私は毎朝、窓が開いていることに気がつきます。
I notice every morning that the window is open.
Nounに気がつく
私はすぐに自分の間違いに気がつく。
I quickly notice my own mistake.
Clause (plain form)ことに気がつく
私は毎朝、窓が開いていることに気がつく。
I notice every morning that the window is open.