eki de saifu o wasuretakoto ni kizukimashita.

This is a very common pattern:

• Vた + ことに気づく
• meaning: to realize / notice that ...

So here:

• 財布を忘れた = forgot my wallet / left my wallet behind
• 財布を忘れたこと = the fact that I forgot my wallet
• 財布を忘れたことに気づきました = I realized that I had forgotten my wallet

The こと turns the whole previous clause into a noun-like phrase, so it can become the thing you noticed.

Because 気づく normally uses に.

• 〜に気づく = to notice / realize something
• not usually 〜を気づく

So:

• 財布を忘れたことに気づきました = I realized that I had forgotten my wallet

You can think of に here as marking the thing your awareness landed on.

Because the forgetting happened before the realizing.

Japanese often uses the past form inside a clause like this when that action is already completed:

• 財布を忘れた = I forgot my wallet
• 財布を忘れたことに気づいた = I realized that I had forgotten my wallet

In English, we often use had forgotten. Japanese does not need a separate past perfect form; the past form plus context is enough.

Yes, こと literally often means thing/matter, but in grammar it frequently works as a nominalizer.

That means it turns a whole sentence or clause into something noun-like.

So:

• 財布を忘れた = I forgot my wallet
• 財布を忘れたこと = the fact that I forgot my wallet

In this sentence, こと is best understood as the fact that.

Yes. You can also say:

• 財布を忘れたのに気づきました

Here, の also nominalizes the clause.

The difference is roughly:

• こと = a bit more formal, objective, or neutral
• の = a bit more conversational

One important note: in 忘れたのに気づきました, this is not the conjunction 〜のに meaning although/even though. It is:

• 忘れたの
  • に気づきました

So it still means I realized that I had forgotten it.

With objects like 財布, 忘れる often means more than just mentally forgetting.

It can mean:

• forgetting to bring something
• leaving something behind somewhere

So 財布を忘れた can naturally mean:

• I forgot my wallet
• I left my wallet behind

In real life, that is usually the idea here: you realized you do not have your wallet with you.

で marks the place where an action happens.

So 駅で means:

• at the station
• in the station depending on context

In this sentence, the most natural reading is that 駅で tells you where the realization happened:

• At the station, I realized that I had forgotten my wallet.

This is a very good question.

In this sentence, the most natural reading is:

• I realized it at the station

Why? Because:

• で marks the place where an action happens
• 気づきました is the main action of the sentence

Also, if you want to say you left your wallet at the station, Japanese often uses に instead:

• 駅に財布を忘れた
• or even more clearly, 駅に財布を置き忘れた

So 駅で財布を忘れたことに気づきました is most naturally understood as:

• At the station, I realized that I had forgotten my wallet.

Because 忘れる is a transitive verb, and the thing forgotten is its direct object.

So:

• 財布を忘れる = to forget one’s wallet

That is why 財布 takes を.

In casual speech, people often drop the particle:

• 財布忘れた

But in a full standard sentence, を is the normal choice.

Japanese often leaves out the subject when it is already clear from context.

So this sentence does not explicitly say I, but it is usually understood as:

• I realized that I had forgotten my wallet at the station

If needed, you could add it:

• 私は駅で財布を忘れたことに気づきました。

But in many situations, that would sound unnecessary.

気づきました is the polite past form.

• dictionary form: 気づく
• plain past: 気づいた
• polite past: 気づきました

So:

• 駅で財布を忘れたことに気づいた。 = plain / casual
• 駅で財布を忘れたことに気づきました。 = polite

The meaning is the same; the difference is just speech level.

Because 気づく is the natural verb for suddenly becoming aware of something you had not noticed before.

• 気づく = notice, realize, become aware
• 分かる = understand, know, figure out

When you suddenly notice Oh no, I forgot my wallet, 気づく is the best fit.

So this sentence sounds very natural with 気づきました.