yokusitu no mado wo akeru to, kazetoosi ga yoku narimasu.

の links two nouns. Here, 浴室の窓 means the bathroom window or more literally the window of the bathroom.

• 浴室 = bathroom / bath room
• 窓 = window

So A の B often means:

• B of A
• A’s B
• B related to A

In this sentence, 浴室の窓 is simply the window that belongs to or is in the bathroom.

を marks the direct object of the verb. The thing being opened is 窓, so it takes を.

• 窓を開ける = to open the window

This is very common:

• ドアを開ける = open the door
• 本を読む = read a book
• 水を飲む = drink water

So in this sentence, 窓 is the thing the action is done to.

When と is used to mean when or if, the verb before it is usually in plain form, not ます form.

So:

• 開けると = when you open it / if you open it

This pattern often describes:

• a natural result
• a regular consequence
• something that happens automatically

Examples:

• 春になると、暖かくなります。 = When spring comes, it gets warm.
• このボタンを押すと、ドアが開きます。 = If you press this button, the door opens.

So 開けると does not sound casual here; it is just the normal grammar pattern.

No. Here と does not mean and.

In this sentence, と means something like:

• when
• if
• once

So:

• 浴室の窓を開けると、風通しがよくなります。
• When you open the bathroom window, the ventilation gets better.

This と often suggests a predictable result. It is not usually used for a speaker’s intention or command after it.

For example, this sounds natural:

• 窓を開けると、涼しくなります。
  When you open the window, it gets cooler.

But this would sound odd:

• 窓を開けると、出かけましょう。
  Because と is not normally used when the second part is a suggestion like let’s go out.

風通し means airflow, air circulation, or ventilation.

Literally:

• 風 = wind / air
• 通し comes from 通す / 通る, related to passing through

So 風通し is the idea that air can pass through a space well.

In daily Japanese, 風通しがいい / 悪い is very common:

• 風通しがいい = well ventilated
• 風通しが悪い = poorly ventilated

It can also be used figuratively in business or social situations:

• 風通しのいい職場 = a workplace where communication is open

But in this sentence it is literal: actual airflow in the bathroom.

が marks the thing that is in the state being described.

Here, the sentence says that the ventilation becomes good, so 風通し is what changes state:

• 風通しがよくなる = the airflow gets better

A useful way to think about it:

• を marks what you directly act on: 窓を開ける
• が marks what is in the condition or what changes: 風通しがよくなる

So the sentence has:

• action on the window
• resulting change in the ventilation

Because いい is an い-adjective, and when an い-adjective connects to なる, the い changes to く.

Pattern:

• Aい → Aくなる = become A

Examples:

• 高い → 高くなる = become expensive / high
• 寒い → 寒くなる = become cold
• よい / いい → よくなる = become better

So:

• 風通しがよくなります = the ventilation becomes good / gets better

Note that いい has an irregular form:

• dictionary form: いい or よい
• adverbial form: よく

That is why it is よくなる, not いいなる.

よい describes a state: is good.
よくなる describes a change: becomes good or gets better.

Compare:

• 風通しがいいです。 = The ventilation is good.
• 風通しがよくなります。 = The ventilation gets better.

In this sentence, opening the window causes a change, so なる is the natural choice.

なります is the polite form of なる.

So:

• よくなる = plain form
• よくなります = polite form

The sentence uses polite style overall, which is very common in explanations, textbooks, and everyday polite speech.

A plain-style version would be:

• 浴室の窓を開けると、風通しがよくなる。

Both mean the same thing, but なります is more polite.

Either can work, depending on context.

Here, と gives a sense of a regular or natural result, so English could be:

• When you open the bathroom window, the ventilation improves.
• If you open the bathroom window, the ventilation improves.

The difference is mostly in English style:

• when sounds like a general fact
• if sounds a little more conditional

Japanese と often sits somewhere between those two in this kind of sentence.

Yes, but the nuance changes a little.

• 浴室 = bathroom / bath room
  More formal, neutral, and common in written or explanatory language.
• 風呂場 = bathroom / bath area
  More everyday and conversational.
• お風呂 usually means bath itself, though depending on context it can sometimes refer to the bathroom area.

So this sentence sounds neat and standard with 浴室.
A more casual everyday version might be:

• 風呂場の窓を開けると、風通しがよくなる。

The comma is there to make the sentence easier to read. It separates the condition from the result:

• 浴室の窓を開けると、 = when you open the bathroom window,
• 風通しがよくなります。 = the ventilation gets better.

Japanese commas are often flexible. In a short sentence like this, the comma is helpful but not absolutely required. You may also see:

• 浴室の窓を開けると風通しがよくなります。

Both are fine.

Yes. That is one of the most natural readings.

Because of 開けると, the sentence sounds like a general statement about what normally happens:

• When you open the bathroom window, the airflow improves.

It does not strongly focus on one specific occasion unless the context makes it specific. So this kind of sentence is often used for:

• explanations
• household advice
• instructions
• general observations

That is why と fits very well here.