〜ことに気づく

With 気づく, に marks what you notice. When you notice a whole fact, make it a noun phrase with こと first. Choose the inner verb form for the fact itself: a completed action can use た, while an ongoing situation can use ている.

1. 1Noun + に気づく
2. 2Verb in plain form + ことに気づく
3. 3Verb in plain form + ことに気づいた

1. 1駅で、切符を家に置いてきたことに気づきました。At the station, I realized that I had left my ticket at home.
2. 2説明を読んで、番号が違っていることに気づいた。Reading the explanation, I noticed that the number was different.

Use に after the thing noticed: 間違いに気づく. A verb clause needs a form such as こと before that に; plain verb + に気づく does not express this construction.

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1. 第7条〜ことTreat an action or fact as a noun phrase
2. 第12条〜ているAn ongoing action, a habit, or a resulting state

• Mango Languages: Nominalizers の and こと ↗ (opens in a new tab)
• Hama Sensei: に with 気づく, question 2(2) ↗ (opens in a new tab)