huyu ha heya ga kansousuru node, neru mae ni hositu wo suru you ni site imasu.

は marks 冬 as the topic: as for winter or in winter.

Here, 冬は sets the seasonal context for the whole sentence. It does not mean winter is the subject of the verb. It is more like:

• As for winter, rooms get dry, so...
• In winter, since the room gets dry...

Japanese often uses は this way to introduce the setting or frame for what follows.

In 部屋が乾燥する, が marks 部屋 as the thing that is doing or undergoing the action/state change of 乾燥する.

So the structure is:

• 部屋が乾燥する = the room gets dry / the room is dry

Using が here is very natural because the sentence is describing what happens in winter: the room dries out.

If you said 部屋は乾燥する, it would sound more like you are contrasting the room with something else, such as:

• The room gets dry, but the bathroom does not

So が is the more neutral choice here.

Literally, yes, it says the room gets dry. But in natural English, what it really refers to is usually the air in the room becomes dry.

Japanese often says 部屋が乾燥する even though an English speaker might expect something like:

• the air in the room gets dry

So this is a normal Japanese way of expressing it.

Both ので and から can mean because / since, but they have a slightly different feel.

• ので sounds softer, more explanatory, and a little more formal
• から can sound more direct or more personal

In this sentence:

• 部屋が乾燥するので = since the room gets dry / because the room gets dry

This fits well because the speaker is calmly giving a reason for a habit.

Compare:

• 乾燥するので、保湿をしています
  softer, explanatory
• 乾燥するから、保湿をしています
  still correct, but a bit more direct

With 前に, Japanese uses the dictionary form of the verb when the action happens before that verb.

So:

• 寝る前に = before sleeping / before I go to bed

This is the normal pattern:

• 食べる前に = before eating
• 出かける前に = before going out
• 寝る前に = before sleeping

By contrast, with 後で / あとで, you often use the past form:

• 寝た後で = after sleeping

So a useful rule is:

• dictionary form + 前に
• past form + 後で

保湿 means moisturizing or keeping moisture in.

As a noun, it often appears with する to make a verb phrase:

• 保湿をする = to moisturize
• 保湿する = also possible, and means the same thing

In this sentence, it probably means something like:

• applying moisturizer
• doing skincare to prevent dryness
• moisturizing the skin, lips, throat, etc., depending on context

So 保湿をする is a very normal noun + する expression.

This is an important nuance.

• 保湿しています = I am moisturizing / I moisturize
• 保湿をするようにしています = I try to make a habit of moisturizing / I make a point of moisturizing

The pattern dictionary form + ようにしている often means:

• making an effort to do something regularly
• trying to keep a certain habit
• intentionally doing something as a practice

So the sentence is not just stating the action. It emphasizes habit and conscious effort.

V dictionary form + ようにしている means:

• to try to do
• to make it a habit to do
• to make a point of doing

Examples:

• 毎日運動するようにしている
  I try to exercise every day.
• 野菜を食べるようにしている
  I make a point of eating vegetables.
• 寝る前に保湿をするようにしている
  I make a habit of moisturizing before bed.

It often implies:

1. the speaker thinks it is a good idea or necessary
2. the speaker does it repeatedly, not just one time

しています is the -te iru form of する.

In this pattern, ようにしている is the normal form when talking about an ongoing habit or repeated effort.

So:

• 保湿をするようにしています = I have been making a habit of moisturizing / I try to moisturize

If you said 保湿をするようにします, it would usually sound more like:

• I will try to moisturize
• a decision from now on

So しています fits better when describing a current routine.

The subject is not stated explicitly, but it is understood from context to be the speaker, probably I.

So:

• 寝る前に = before I go to bed
• 保湿をするようにしています = I try to moisturize

Japanese often omits subjects when they are clear from context. This is very common and natural.

Both are possible, but they focus slightly differently.

• 部屋が乾燥する = the room gets dry
• 空気が乾燥する = the air gets dry

空気が乾燥する is more physically precise, but 部屋が乾燥する is also natural and common in everyday Japanese. It is similar to how English sometimes says the room is dry, even though technically the air is dry.

Yes. Both are possible.

• 保湿をする
• 保湿する

They mean the same thing. In many cases, 保湿する sounds a little more compact, while 保湿をする can sound slightly more explicit.

Japanese often allows both patterns with suru-nouns:

• 勉強する / 勉強をする
• 掃除する / 掃除をする
• 保湿する / 保湿をする

In this sentence, 保湿をするようにしています sounds perfectly natural.

It breaks down like this:

• 冬は = as for winter / in winter
• 部屋が乾燥するので = because the room gets dry
• 寝る前に = before going to bed
• 保湿をするようにしています = I try to make a habit of moisturizing

So the grammar flow is:

topic + reason + time expression + habitual effort

That is a very common Japanese sentence pattern.