eki wo toorisugite simatta node, tugi no singou de hikikaesimasita.

That is a very common question. In this sentence, を is not marking a typical direct object in the English sense. With movement verbs, を can mark a place that someone moves through, along, or past.

So in 駅を通り過ぎる, 駅を means something like:

• past the station
• by the station

It is similar to how 道を歩く means walk along the road.

So here, 駅を通り過ぎてしまった means (I) went past the station / passed the station.

It is made of two parts:

• 通り過ぎる = to go past, to pass by
• 〜てしまう = to finish doing something, or to do something completely; often it also adds a nuance of regret, accident, or something unintended

So:

• 通り過ぎた = passed by
• 通り過ぎてしまった = ended up passing by / accidentally passed by / passed it and that was unfortunate

In this sentence, 〜てしまった strongly suggests that passing the station was not intentional. It has a slight oops feeling.

Because しまう attaches to the て-form of the verb before it.

The pattern is:

• verb in て-form + しまう

So:

• 通り過ぎる → 通り過ぎて
• 通り過ぎてしまう

Then it is put into past tense:

• 通り過ぎてしまった

This is a fixed grammar pattern. You cannot attach しまう directly to the polite ます form.

Not always. It has two main uses:

1. completion
   • the action is fully done
2. unintended / unfortunate result
   • something happened by accident, or in a way the speaker regrets

In this sentence, the second meaning is the important one. Missing your station is usually not planned, so 通り過ぎてしまった sounds natural.

If there were no regret nuance, the sentence could simply say 駅を通り過ぎた, but 〜てしまった makes it sound more like I accidentally went past it.

Both ので and から can mean because, but they feel a little different.

• ので sounds a bit more explanatory, softer, and often more polite
• から can sound more direct or more subjective

So:

• 駅を通り過ぎてしまったので、次の信号で引き返しました。 = Because I had passed the station, I turned back at the next light.

This sounds natural and explanatory, like calmly describing what happened.

If you used から instead, it would still be grammatical:

• 駅を通り過ぎてしまったから、次の信号で引き返しました。

That version is a bit more straightforward and conversational.

次の信号 means the next traffic light / signal.

The particle で marks the place or point at which an action happens.

So:

• 次の信号で引き返しました = turned back at the next traffic light

Here, で tells you the location or situational point where the speaker turned back.

English learners sometimes expect に, but で is more natural here because it marks the place where the action 引き返しました happened.

Normally, no. で is the natural choice here.

Why?

• で = the place where an action occurs
• に = a destination, point of existence, or target of movement

In this sentence, the traffic light is not really the destination in the sense of going to it as a final target. It is the point at which the speaker turned back. So で fits better.

Think of it as:

• at the next light, I turned around

not

• to the next light, I turned around

引き返す means to turn back, to go back the way you came, or to retrace your steps.

It is close to 戻る (to return / go back), but 引き返す often emphasizes reversing direction after having gone too far or deciding to go back from a certain point.

That makes it a very good match here:

• the speaker passed the station
• then, at the next traffic light,
• they turned back

So 引き返しました is slightly more specific and vivid than just 戻りました.

Because the whole sentence is written in polite style.

Compare:

• plain style: 引き返した
• polite style: 引き返しました

The sentence mixes:

• 通り過ぎてしまった (plain past inside the reason clause)
• 引き返しました (polite main predicate)

This is normal. In Japanese, the final verb usually determines the overall politeness of the sentence. The earlier clause before ので does not have to be polite.

So this is perfectly natural Japanese.

The subject is omitted because Japanese often leaves out information that is clear from context.

In English, you usually need to say I, we, he, and so on. In Japanese, if it is obvious who is being talked about, the subject is often not stated.

So this sentence naturally implies something like:

• I passed the station, so I turned back at the next light
• or possibly we passed the station...

The exact subject depends on context.

It breaks down like this:

• 駅を通り過ぎてしまったので = because (I) ended up passing the station

• 次の信号で引き返しました = (I) turned back at the next traffic light

So the full structure is:

• [reason] ので, [main action]

This is a very common Japanese sentence pattern.

In everyday Japanese, 信号 often means traffic light or traffic signal.

Technically, the word can mean signal more generally in other contexts, but in a sentence like 次の信号で, most people will understand it as the next traffic light.

So here that is the natural interpretation.

Yes. A casual version could be:

• 駅を通り過ぎちゃったので、次の信号で引き返した。

or

• 駅を通り過ぎちゃったから、次の信号で引き返した。

A few things changed:

• 〜てしまった → 〜ちゃった
  This is a common casual contraction.
• 引き返しました → 引き返した
  Plain past instead of polite past.

So the original sentence is polite and natural, while these versions sound more casual and conversational.

過ぎる by itself means to pass or to exceed, and it has several different uses depending on context.

通り過ぎる is more specific: it means to go past something, physically passing by it.

In this sentence, the speaker physically went beyond the station, so 通り過ぎる is a very natural choice.

Compare:

• 駅を通り過ぎる = go past the station
• 時間が過ぎる = time passes
• やり過ぎる = overdo something

So 通り過ぎる gives a clear movement image.

No, it is not strictly necessary, but it is very common in writing.

The comma helps show the pause between:

• the reason clause
• the main action

So:

• 駅を通り過ぎてしまったので次の信号で引き返しました。

is still grammatical, but

• 駅を通り過ぎてしまったので、次の信号で引き返しました。

is easier to read.