にきがつく

気がつく means to become aware of something or realize a fact, often after noticing a change or receiving new information. The particle に marks what you become aware of. There are two common patterns: (1) Nに気がつく for noticing a concrete fact or situation (e.g., a cat on the roof); (2) ことに気づく for realizing a fact about a situation or someone’s behavior, often expressed as ことに気づく with the verb in the plain form plus こと (e.g., 気づくことに). Use 〜ことに気づく to express realization that something is the case, sometimes preceded by a change in understanding. Notes: 気づく is often written as 気がつく in informal writing; the meaning remains the same regardless of kanji choice.