tuyu ha musiatui hi ga ooi node, kankisen wo yoku tukaimasu.

This is just the standard Japanese reading of the word for the rainy season: 梅雨（つゆ）.

A lot of Japanese words use special or less predictable readings, especially for common seasonal words. So this is something you usually just memorize as a set:

• 梅雨（つゆ） = rainy season

You may also sometimes see ばいう in formal contexts like weather reports, but つゆ is the everyday reading.

は marks 梅雨 as the topic of the sentence.

So the sentence is framed as As for the rainy season... or During the rainy season...

Here, 梅雨は does not mean the rainy season is literally doing the action. It sets the overall context, and then the sentence says what is true in that context:

• 梅雨は = as for the rainy season / during the rainy season
• 蒸し暑い日が多い = there are many hot-and-humid days
• ので = so / because
• 換気扇をよく使います = I/we often use the ventilation fan

Because 蒸し暑い is an i-adjective, and when an i-adjective directly modifies a noun, it stays in its plain form:

• 蒸し暑い日 = hot-and-humid days

You use 蒸し暑く in other grammatical situations, such as before another adjective-like element or in adverbial use, but not directly before a noun.

Compare:

• 暑い日 = a hot day
• 蒸し暑い日 = a humid, muggy day

So 蒸し暑い日 is the normal noun-modifying form.

Because が marks 日 as the thing that is numerous.

In Japanese, 多い often works with the pattern:

• X が多い = there are many X / X are numerous

So:

• 蒸し暑い日が多い = there are many hot-and-humid days

If you used 日は, it would sound more contrastive, like as for the days... and would change the focus. Here, the sentence simply states that the number of muggy days is high, so が is the natural choice.

In Japanese, 多い literally means many / numerous, but it often functions in a way that English expresses with there are many.

So:

• 蒸し暑い日が多い literally: hot-and-humid days are many natural English: there are many hot-and-humid days

This is a very common Japanese pattern:

• 雨の日が多い = there are many rainy days
• 人が多い = there are a lot of people
• 質問が多い = there are many questions

Both can mean because, but ので usually sounds a bit softer, more explanatory, and a little less direct than から.

Here:

• 蒸し暑い日が多いので、換気扇をよく使います。 = Because there are many muggy days, I often use the ventilation fan.

If you used から:

• 蒸し暑い日が多いから、換気扇をよく使います。

that would still be correct, but it sounds a bit more straightforward or conversational.

A simple way to think of it:

• ので = since / because, with a slightly gentler explanatory feel
• から = because, more direct

換気扇（かんきせん） means a ventilation fan or exhaust fan.

It is the kind of fan that removes air, moisture, smells, or steam from a room. Depending on the situation, in English it could refer to:

• a kitchen exhaust fan
• a bathroom ventilation fan
• a fan used to improve air circulation indoors

In this sentence, because the weather is humid, the idea is that the speaker often uses the ventilation fan to reduce stuffiness or moisture.

Because 使う is a transitive verb, and the thing being used is marked with を.

So:

• 換気扇を使います = use the ventilation fan

This is the normal pattern:

• ペンを使う = use a pen
• 日本語を使う = use Japanese
• エアコンを使う = use the air conditioner

Here, 換気扇 is the object of 使います, so it takes を.

よく means often in this sentence.

It comes before the verb because it is an adverb modifying 使います:

• よく使います = often use

This placement is very normal in Japanese. Adverbs usually come before the word or phrase they modify.

Be careful: よく can also mean well, depending on context.

For example:

• 日本語がよくわかります = I understand Japanese well
• この店によく行きます = I often go to this shop

In your sentence, often is the natural meaning.

Japanese often omits the subject when it is understood from context.

So 使います could mean:

• I use it
• we use it
• people use it

depending on the situation.

In a standalone textbook sentence, it is often understood as I often use the ventilation fan or we often use the ventilation fan. Japanese does this very frequently, and leaving out the subject sounds natural when it is obvious or not important.

The comma separates the reason part from the main statement:

• 蒸し暑い日が多いので、 = because there are many muggy days,
• 換気扇をよく使います。 = I often use the ventilation fan.

It helps readability and gives a slight pause. In Japanese, punctuation can be somewhat flexible, but this comma is very natural after a longer reason clause ending in ので.

Yes, that is a good way to understand the structure.

Japanese often says something like:

• X は Y が多い

which can feel like:

• As for X, there are many Y
• or X has many Y

So:

• 梅雨は蒸し暑い日が多い = During the rainy season, there are many muggy days = The rainy season has many muggy days

The natural English translation depends on style, but grammatically that is the idea.