Learn Hardcore Japanese: eki no esukareetaa ga konde iru toki ha, tuuro no hasi wo aruite kaidan made iku. - When the station escalator is crowded, I walk along the edge of the aisle to the stairs.

駅のエスカレーター emphasizes which escalator: “the station’s escalator” as a specific thing that belongs to or is located in the station.
If you said 駅でエスカレーターが混んでいる, 駅で would mean “at the station”, focusing on the place where the escalator is crowded, rather than specifying which escalator.

Both ideas are possible in Japanese, but X の Y is the standard way to say “the Y of X” or “the Y at X,” so 駅のエスカレーター is the natural choice as a noun phrase here.

Why is が used with エスカレーター (エスカレーターが混んでいる) instead of は?

が marks the grammatical subject of the clause エスカレーターが混んでいる:

エスカレーターが混んでいる = “the escalator is crowded.”

This whole clause modifies とき:

エスカレーターが混んでいるとき = “when the escalator is crowded.”

Inside a 〜とき clause, が is typically used to mark the subject of that clause.
If you said エスカレーターは混んでいるとき, it would sound odd because:

は usually marks the topic of the main sentence.
Here, とき is functioning more like part of a subordinate clause, not the main topic holder; the main topic is actually “when that happens” (marked by ときは).

So: エスカレーターが混んでいるとき is the normal, natural structure.

What exactly does 混んでいる mean here? Why not just 混んだ?

混んでいる is the ている form of 混む (“to become crowded”), but in this case it expresses a resulting state, not an ongoing action.

混む – to get crowded (the action of people filling up).
混んでいる – is in a crowded state (has become crowded and remains so).

So エスカレーターが混んでいる = “the escalator is crowded.”

If you said 混んだエスカレーター, that’s grammatically possible, but:

混んだ often focuses more on the completed action “got crowded,” and as an adjective before a noun it can sound a bit stiffer or less natural in everyday speech.
For “is crowded (now),” 混んでいる is the most natural.

What is the function of ときは? Why ときは and not ときに?

とき means “when” and turns the preceding clause into a time expression:

エスカレーターが混んでいるとき = “when the escalator is crowded.”

Then:

は after とき turns that time into the topic:
エスカレーターが混んでいるときは、…
= “As for the times when the escalator is crowded, …”

If you used に as in エスカレーターが混んでいるときに, it would mean:

“At the time when the escalator is crowded, (something happens)” – focusing on time as a point when an event occurs.

Here, the speaker is stating a general rule / habitual action for those situations, so making it a topic with は is more natural:
“When the escalator is crowded, (I habitually do X).”

Why is there no subject like 私 in the second part? Who is walking?

Japanese often omits the subject when it’s clear from context.

通路の端を歩いて階段まで行く。

In isolation, this could mean “I walk,” “you walk,” “people walk,” etc. But in a real conversation or text, context usually tells you:

If the speaker is talking about their own habit → it means “I.”
If it’s a sign / rule → it might mean “people (in general) should.”

Since this sentence describes what someone habitually does when the escalator is crowded, the implied subject is usually “I” or “we,” depending on the context. Japanese leaves that unstated because it’s obvious in the situation.

Why is it 通路の端を歩いて and not 通路の端に歩いて or 通路の端で歩いて?

Here を after 通路の端 marks the path/route that you move along.

通路の端を歩く = “walk along the edge of the passageway.”

Key patterns:

場所を歩く / 通る / 走る → を can mark the space you move through.
e.g. 公園を歩く – walk through the park.

If you used:

通路の端に歩く – ungrammatical / unnatural; に doesn’t work with 歩く this way.
通路の端で歩く – “walk at the edge of the passageway” (focusing on location, not the route). But the meaning here is “use that edge as a route,” so Japanese uses を.

So 通路の端を歩いて is: “by walking along the edge of the passageway.”

What is the role of 歩いて before 階段まで行く? Why say both 歩いて and 行く?

歩いて is the て-form of 歩く, used to connect verbs.

Here, 歩いて行く is a very common pattern:

歩いて行く = “go (there) by walking / walk to (there).”

So:

通路の端を歩いて階段まで行く。
= “(I) walk along the edge of the passageway and go to the stairs.”
More naturally in English: “(I) walk along the edge of the passageway to the stairs.”

The て-form can show:

Sequence: “do X and then Y.”
Manner/means: “do Y by doing X.”

Here it’s mainly means: you go to the stairs by walking along the edge.

Why use まで after 階段 (階段まで行く) instead of に or へ?

まで means “up to” / “as far as” and emphasizes the end point / limit of movement:

階段まで行く = “go as far as the stairs / go up to the stairs.”

Comparison:

階段に行く / 階段へ行く
Grammatically possible, but less common and can sound slightly unnatural here. に and へ simply mark the destination “to the stairs,” without that “up to there” nuance.

In practice:

For distances / end points you’re moving along, まで is very natural.
It fits nicely with the idea of walking along a passage up to the stairs.

What does 通路 mean exactly? Why not use 道 or 通り?

All three are related to “roads/paths,” but they differ:

通路 – “passageway, corridor, passage.”
Used for indoor or enclosed walkways: in stations, buildings, planes, etc.
道 – “road, way, path.”
Very general: roads, paths, routes (both literal and metaphorical).
通り – “street, avenue, (a named) road.”
Often for city streets where cars and people pass.

Since this is inside a station, the indoor walkway is best called 通路.
通路の端 = “the edge/side of the passageway (corridor).”

How do you read 端 in 通路の端 and what nuance does it have?

In this context, 端 is read はし (hashi).

通路の端 = つうろのはし
Meaning: “the edge,” “the side,” “the end” of the passageway.

Nuance:

It suggests the side area, not the middle where most people walk or stand.
It doesn’t mean a physical “end point” only; it can also refer to a side edge.

So the sentence suggests: “keep to the side” of the corridor to walk to the stairs.

Why is the sentence ending with the plain form 行く and not 行きます? Is this casual?

Yes, 行く is the plain (dictionary) form; 行きます is the polite form.

行く – used in casual speech, inner monologue, narration, explanations in textbooks, and when giving general example sentences.
行きます – used in polite speech toward others.

So:

In a textbook explanation or grammar example, 行く is common.
In actual polite conversation, you would probably say:
エスカレーターが混んでいるときは、通路の端を歩いて階段まで行きます。

The grammar is the same; only the politeness level changes.