Learn Hardcore Japanese: zangyou de kaeri ga osoi hi ha, senpuuki wo tukete heya wo suzusiku sinagara, reitousyokuhin wo atatamete kantan ni bangohan wo sumasemasu. - On days when I come home late because of overtime, I turn on the fan to cool the room and just heat up frozen food to finish dinner simply.

残業に帰りが遅い日 – ungrammatical here.
残業の日 – “days of overtime / overtime days” (focuses on the day being an overtime day, not explicitly on the lateness of the return).

So 残業で clearly explains why the return is late: “On days when I get home late because of overtime …”

In 帰りが遅い日, what exactly is 帰り? Why not 帰るのが遅い日?

帰り is a noun meaning “return (home) / the act of going back / the way back”.

帰る – to go home, to return
帰り – the return, one’s way back

So:

帰りが遅い日
literally: “days when (my) return is late”
= “days when I get home late”

You could also say:

帰るのが遅い日

This is also natural and means essentially the same thing: “days when (I) get home late”.

Nuance:

帰りが遅い日 – slightly more compact and nominal: “my return is late”.
帰るのが遅い日 – more directly “the act of returning is late”.

Both are fine in everyday Japanese; 帰りが遅い日 is very common and idiomatic.

How does 帰りが遅い日は work grammatically? What is 日 doing, and why is there a は after it?

The structure is:

[帰りが遅い] 日は …

This is:

A relative clause:
- 帰りが遅い (my return is late) modifies 日 (day).
  → “days when my return is late / days when I get home late”.
Then 日 is followed by は, which marks that kind of day as the topic of the sentence:
- 帰りが遅い日は、…
  “As for days when I get home late, …”

So the sentence means:

As for days when I get home late because of overtime, (I do the following…).

Why are only the last verb すませます in polite form, while the others are in て-form or plain form? Is that normal?

Yes, that’s normal and very important to understand.

In Japanese, when you string actions together with 〜て or 〜ながら, you usually:

Put only the final main verb in polite form (ます／です).
The preceding verbs appear in connecting forms (て-form, ながら, etc.), not in polite form.

Here:

扇風機をつけて
部屋を涼しくしながら
冷凍食品を温めて
簡単に晩ご飯をすませます。

All of these share the same subject (“I”), and すませます is the main, final action. The earlier parts are like “doing X, doing Y, doing Z, and then I finish dinner…”.

You generally don’t say:

✕ 扇風機をつけます、部屋を涼しくします、冷凍食品を温めて、簡単に晩ご飯をすませます。

You could say it, but it sounds choppy and list-like. The て／ながら style makes it one smooth sentence.

In 扇風機をつけて, what does つける mean here, and how is it different from つく?

Here つける is a transitive verb meaning “to turn on / switch on” (a device, light, etc.).

扇風機をつける – to turn on the fan
電気をつける – to turn on the light
テレビをつける – to turn on the TV

The intransitive counterpart つく means “to go on / be on”:

扇風機がついている – the fan is on
電気がつく – the light comes on

In the sentence:

扇風機をつけて…
→ “I turn on the fan and then… / having turned on the fan, …”

How does 部屋を涼しくしながら work? What is the grammar behind 涼しくする?

Two points here:

Adjective + く + する pattern
- 涼しい (cool) → 涼しく (adverbial / -く form)
- 涼しくする = “to make (something) cool”

With を:

部屋を涼しくする – to make the room cool

This pattern is very common:

部屋をきれいにする – make the room clean
音を大きくする – turn up the volume (make sound big)
部屋を静かにする – make the room quiet

〜しながら
- する
  - ながら → “while doing (something)”

So:

部屋を涼しくしながら
literally: “while making the room cool”
→ “while cooling the room”

Putting it together with the previous part:

扇風機をつけて部屋を涼しくしながら、…
“turning on the fan and, while cooling the room, …”

What exactly does 〜しながら express here, and how is it different from using just 〜て?

〜しながら means “while doing ~”, implying two actions happening at the same time, with the same subject.

In the sentence:

部屋を涼しくしながら、冷凍食品を温めて…

The speaker is:

cooling the room
heating up frozen food

at (roughly) the same time.

Differences:

AしながらBする
→ “do B while doing A” (simultaneous)
AしてBする
can be:
- sequence: do A and then B
- or looser connection: A and B both happen, but not necessarily at the same time.

Here, ながら highlights that cooling the room is a background/parallel action while the main thing is getting dinner ready with frozen food.

In 冷凍食品を温めて, why is 温める used here, and how is it different from 暖める?

Both 温める and 暖める are read あたためる, but the kanji usage typically differs:

温める – to warm up something like food, drink, bathwater; also “to warm up” in a more neutral sense.
- ご飯を温める – warm up rice
- スープを温める – heat soup
暖める – to warm up in the sense of temperature providing warmth/comfort: body, room, feelings.
- 部屋を暖める – warm up a room
- 体を暖める – warm the body

In this sentence:

冷凍食品を温めて
→ “heating up frozen food”

Since it’s food, 温める is the natural choice. You may see mixing in real life, but this is the standard distinction taught.

Why is it 簡単に and not 簡単な or 簡単で? What is 簡単に modifying?

簡単 is a な-adjective. To use it as an adverb (to modify a verb), you change な to に:

簡単な仕事 – an easy job (adjective)
簡単に終わる – to finish easily / simply (adverb)

In the sentence:

簡単に晩ご飯をすませます。

簡単に modifies すませます:

“I simply / easily / in a simple way get dinner over with.”

So:

簡単な晩ご飯をすませます would mean “I finish a simple dinner” (describing the dinner).
簡単に晩ご飯をすませます means “I deal with dinner in a simple way” (describing the manner of the action).

Here the idea is “I just take the easy way out for dinner by heating up frozen food.”

What does 晩ご飯をすませます really mean? How is すませる different from just saying 晩ご飯を食べます?

すませます is the polite form of 済ませる / 済ます, meaning:

“to finish, to get something over with, to settle something”

The phrase 〜をすませる often implies:

You take care of some necessary task,
often quickly / with minimal fuss, or in a somewhat practical, not-special way.

Examples:

宿題をすませる – get your homework done
朝ご飯をすませる – just have breakfast (often something quick), get breakfast out of the way
用事をすませる – take care of some errands

So:

晩ご飯を食べます – I eat dinner. (neutral)
晩ご飯をすませます – I get dinner over with / just take care of dinner.

With the rest of the sentence (冷凍食品を温めて簡単に), the nuance is:

I don’t cook a proper meal; I just heat up some frozen food and call that dinner, getting it over with simply.

There are many を in this sentence: 扇風機を, 部屋を, 冷凍食品を, 晩ご飯を. Is that okay? How does the listener know which を goes with which verb?

It’s perfectly normal. Each を attaches to the closest suitable verb within its verb phrase:

扇風機をつけて – turn on the fan
部屋を涼しくしながら – make the room cool
冷凍食品を温めて – heat frozen food
晩ご飯をすませます – finish dinner / get dinner over with

Because Japanese word order is [object] を [verb], and clauses are chained with て and ながら, it’s clear which object belongs to which verb.

Native speakers parse it as four mini-phrases, not as one big confusing chain.

The subject “I” never appears in the Japanese sentence. How do we know it means “I turn on the fan… I heat frozen food… I finish dinner…”?

Japanese often omits the subject when it’s clear from context. In this kind of everyday, first-person description about daily routine, the default assumption is that the subject is the speaker.

From context:

Talking about “overtime” and “days when [my] return is late”
Describing what one does at home regarding dinner

It naturally reads as:

“On days when I get home late because of overtime, I turn on the fan, I cool the room, I heat up frozen food, and I quickly finish dinner.”

If the subject needed to be someone else, Japanese would add it explicitly (e.g., 父は, 彼女は, etc.). Since no explicit subject appears, and this sounds like a personal routine, “I” is the natural interpretation.

Does this sentence describe a general habit, or one specific day? How does Japanese show that it’s a regular pattern?

This sentence describes a habitual action / routine, not a one-time event. Clues:

日は – “(on) days when…”
- 帰りが遅い日は sets up a general condition: “As for days when I get home late, …”
- It’s about a type of day, not a specific date.
The verb すませます is in non-past (present/future) form:
- Japanese non-past can express generic or habitual actions:
- 毎朝コーヒーを飲みます。 – I (usually) drink coffee every morning.
- Similarly, here: “On days when I get home late, I (usually) just…”

So the natural reading is:

On days when I get home late because of overtime, I (tend to) just cool the room with a fan and quickly get dinner over with by heating up frozen food.