huyu ha heya ga kansousuru node, kasituki wo tuketa mama nemasu.

Here 冬は sets the overall topic or time frame: as for winter or more naturally in winter.

Japanese often uses a season word plus は to mean speaking of that season / during that season in general. You do not need に here, because this is not a single specific point in time like 3時に or 月曜日に. It is a broad seasonal context.

So 冬は is very natural for:

• 冬は寒いです。 = Winter is cold / It’s cold in winter.
• 冬は部屋が乾燥する。 = In winter, rooms get dry.

Because 部屋 is the thing that becomes dry.

The verb 乾燥する here is intransitive in meaning: to become dry / to dry out. So the room is the grammatical subject of that verb, and が marks that subject.

• 部屋が乾燥する = The room gets dry / the room dries out

If you used 部屋は, that would make room the topic instead:

• 部屋は乾燥する = As for the room, it gets dry

That is possible in some contexts, but in this sentence 冬は is already the main topic, so 部屋が works well to show what specifically gets dry within that winter context.

Yes. 乾燥する is a suru-verb built from the noun 乾燥 meaning dryness.

As a verb, it means:

• to dry out
• to become dry
• sometimes to dry depending on context

In this sentence, it means the room becomes dry or gets dry in winter.

So:

• 部屋が乾燥する = The room gets dry

This is a general/habitual statement, not a description of one exact moment.

Both can give a reason, but ので sounds a bit more explanatory, neutral, and softer.

• Aので、B = Because A, B
• Aから、B = also Because A, B, but often a bit more direct or subjective

So:

• 部屋が乾燥するので、加湿器をつけたまま寝ます。 feels like a calm explanation: Since the room gets dry, I sleep with the humidifier on.

If you changed it to から, the meaning would still be very similar:

• 部屋が乾燥するから、加湿器をつけたまま寝ます。

That is also natural, just slightly less soft/formal in tone.

Because つける is a transitive verb here, meaning to turn on.

The thing being turned on takes を:

• 電気をつける = turn on the light
• テレビをつける = turn on the TV
• 加湿器をつける = turn on the humidifier

So 加湿器を is the direct object of つけた.

Also, the reading of 加湿器 is かしつき.

まま means leaving something as it is or remaining in that state.

The pattern is:

• verb in ta-form + まま

So:

• つけたまま = left on
• literally: in the state of having turned it on

Therefore:

• 加湿器をつけたまま寝ます means I sleep with the humidifier left on.

Very common examples:

• ドアを開けたまま = with the door left open
• 電気をつけたまま = with the light left on
• 座ったまま = remaining seated

Because with まま, Japanese usually uses the ta-form when you mean after doing something, that resulting state continues.

So:

• つけたまま = having turned it on, leaving it in that state
• 開けたまま = having opened it, leaving it open

The ta-form here does not mainly indicate past time. It indicates a completed action whose result is still true.

That is why つけたまま寝ます means:

• I sleep while it remains on not
• I turned it on in the past and then slept in a strongly past-time sense

Because this sentence describes a general habit or usual action.

• 寝ます here means I sleep / I go to bed
• 寝ています would usually describe an ongoing state at a particular time, like is sleeping or is asleep

So:

• 加湿器をつけたまま寝ます = I sleep with the humidifier on
  → a routine or general practice

If you said:

• 加湿器をつけたまま寝ています it would sound more like:
• I am sleeping with the humidifier on which is about an ongoing situation, not a normal habit statement.

Yes, the subject is omitted because it is understood from context.

A natural full version could be:

• 私は冬は部屋が乾燥するので、加湿器をつけたまま寝ます。

But Japanese often leaves out 私は when it is obvious that the speaker is talking about their own habit.

Omitting the subject makes the sentence sound natural and not repetitive.

Yes, it could, but the nuance changes a little.

• 部屋が乾燥する focuses on the room as the thing that gets dry
• 部屋は乾燥する would make the room a topic, often with a contrastive feel, like as for the room, it gets dry

In this sentence, 冬は is already the main topic, so 部屋が is the cleaner, more natural choice for many contexts.

So 部屋が乾燥するので is a very standard way to say:

• since the room gets dry

Yes, they are similar, but the nuance is different.

• つけたまま = left on, neutral description
• つけっぱなし = left on and not turned off, often with a nuance of carelessness, wastefulness, or negligence

So:

• 加湿器をつけたまま寝ます = neutral, just describing how you sleep
• 加湿器をつけっぱなしで寝ます = can sound a little more like I leave it running

In your sentence, つけたまま is the safer and more neutral choice.

It breaks down like this:

• 冬は = in winter / as for winter
• 部屋が乾燥するので = since the room gets dry
• 加湿器をつけたまま = with the humidifier left on
• 寝ます = I sleep / go to bed

So the grammar pattern is basically:

[time/topic] + [reason clause] + [state maintained] + [main action]

That pattern is very common in Japanese.