Learn Hardcore Japanese: natu no yoru ni ha, senpuuki no oto wo kikinagara nemasu. - On summer nights, I sleep while listening to the sound of the fan.

夏の夜に寝ます。
= I sleep on summer nights. (Plain statement; just saying when.)
夏の夜には寝ます。
= As for summer nights, I sleep. (Topical or slightly contrastive; e.g. in summer nights (at least), I sleep / compared to other times.)

In this sentence, 夏の夜には、扇風機の音を聞きながら寝ます。, には suggests something like:

When it’s summer nights (as opposed to other times), I sleep while listening to the fan.
It feels a bit like you’re highlighting summer nights as a special time with this habit.

What is the function of の in 夏の夜 and 扇風機の音?

の here is a possessive / attributive marker.

夏の夜
Literally: summer’s night, i.e. summer night (a night that belongs to or is associated with summer).
扇風機の音
Literally: fan’s sound, i.e. the sound of the fan.

So A の B often corresponds to English B of A or A B:

日本の文化 → Japanese culture
犬の声 → the sound of a dog / a dog’s bark

Why is 音 marked with を (扇風機の音を) and not が?

を marks the direct object of a verb.
The verb is 聞きます (to listen / to hear).
You listen to / hear something, so that something is the object:

音を聞く → to listen to / hear a sound

が usually marks the subject or the thing that exists/appears, etc. Using 音が would make the sound the subject, which doesn’t fit as well with 聞く in this pattern.

So:

扇風機の音を聞きます。
I listen to the sound of the fan. (Sound = object)

If you used が, you’d need a different verb or structure, like:

扇風機の音が聞こえます。
I can hear the sound of the fan. (Sound = thing that is audible)

What does 聞きながら mean exactly, and what is the grammar pattern?

聞きながら means while listening.

Grammar pattern:

Take the ます-stem (the verb stem used before ます)
Add ながら

For 聞きます:

Dictionary form: 聞く
ます-form: 聞きます
Stem: 聞き
Stem + ながら: 聞きながら → while listening

General pattern:

食べます → 食べ
- ながら → 食べながら (while eating)
歩きます → 歩き
- ながら → 歩きながら (while walking)

In the sentence:

扇風機の音を聞きながら寝ます。
I sleep while listening to the sound of the fan.

So V-stem + ながら + main verb expresses: do the main verb while doing V (at the same time).

What is the difference between 聞きながら寝ます and 聞いて寝ます?

Both can be translated as sleep while listening, but the nuance is slightly different:

聞きながら寝ます
Emphasizes simultaneity: sleeping and listening are happening at the same time in a continuous way.
→ I sleep as I listen to the sound.
聞いて寝ます (て-form + 寝ます)
Is less strictly “at the same time.” Depending on context, it can feel like:
- I listen (and then) sleep.
- I sleep with the sound playing.

For clear “two things at the same time,” V-stem + ながら is the more precise pattern. So 聞きながら寝ます is textbook “sleep while listening.”

Why is the verb 寝ます in the polite non-past form? Is this present or future?

Japanese non-past form (寝ます) covers:

Present habits / general truths
Future actions

In this sentence, it describes a habitual action:

夏の夜には、扇風機の音を聞きながら寝ます。
→ On summer nights, I sleep while listening to the fan. (This is what I usually do.)

If you wanted to clearly mark a specific past event, you’d use:

寝ました → slept

For example:

きのうの夜、扇風機の音を聞きながら寝ました。
Last night, I slept while listening to the sound of the fan.

Why is there no explicit subject like 私は in the sentence?

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

In English, you must say I sleep.
In Japanese, if it’s clear we’re talking about the speaker’s own habits, 私は is unnecessary:

(私は) 夏の夜には、扇風機の音を聞きながら寝ます。

Adding 私は is not wrong, but it can sound a bit heavy or overly explicit if context already makes it clear that I am the subject. Native speakers tend to drop it unless they need to emphasize I specifically or contrast with someone else.

Could you say 夏の夜で instead of 夏の夜に?

For time expressions, Japanese normally uses に, not で.

夏の夜に寝ます。 ✅
I sleep on summer nights.

で is more for:

Location of an action:
学校で勉強します。 → I study at school.
Means / method:
バスで行きます。 → I go by bus.

With times like 月曜日, ３時, 夏の夜, use に:

月曜日に行きます。
３時に起きます。
夏の夜に寝ます。

Can the word order be changed, like putting 扇風機の音を at the beginning?

Yes, Japanese word order is relatively flexible as long as you keep particles attached correctly. The main verb usually comes at the end.

Some possible variations:

夏の夜には、扇風機の音を聞きながら寝ます。 (original)
扇風機の音を聞きながら、夏の夜には寝ます。
扇風機の音を、夏の夜には聞きながら寝ます。

All are grammatical, but the focus changes slightly depending on what you place earlier. The original puts the time frame (夏の夜には) up front, which is very natural when describing a habit tied to a certain time.

Is 扇風機 the only way to say fan? What about an electric fan?

In modern Japanese, 扇風機 commonly means an electric fan (the household appliance that blows air).

Historically:

扇 = hand fan
風 = wind
機 = machine

So 扇風機 is literally a machine that makes fan-like wind, and in everyday usage it refers to an electric fan.

If you mean a handheld fan, you’d usually say:

うちわ → round paper/plastic hand fan
扇子（せんす） → folding fan

But in this sentence, 扇風機 is very naturally understood as a typical electric fan used at night in summer.