ame no hi ni mado wo akeppanasi ni suru to, kaaten ga nurete simaimasu.

雨の日 literally means a day of rain, which is the Japanese way to say a rainy day.

• 雨 = rain
• 日 = day
• の links the two nouns

So 雨の日 is a noun phrase: rainy day.

Then に marks the time/context in which something happens. In this sentence, 雨の日に means on a rainy day or on rainy days.

The に here marks a time expression.

In Japanese, some time words can take に to mean at/on/in that time.
So:

• 雨の日に = on a rainy day
• 日曜日に = on Sunday
• 夏に = in summer

Here, it sets the situation for the rest of the sentence: on a rainy day, if you leave the window open...

開けっぱなし means left open or left open and not closed.

It comes from:

• 開ける = to open
• 〜っぱなし = left in that state; done and then not fixed/finished properly

So 開けっぱなし often has the nuance of carelessly leaving something as it is.

Examples:

• 電気をつけっぱなし = leaving the lights on
• ドアを開けっぱなし = leaving the door open

In your sentence, 窓を開けっぱなし means leaving the window open.

開けっぱなし by itself is a state: left open.

Adding にする means to make it that way / to leave it in that state.

So:

• 窓を開けっぱなし = the window is left open / an open-left state
• 窓を開けっぱなしにする = to leave the window open

This pattern is common:

• 静かにする = make it quiet / be quiet
• そのままにする = leave it as it is
• 開けっぱなしにする = leave it open

That に marks the state that something is being put into or kept in.

Think of it like this:

• 開けっぱなし = the state of being left open
• にする = to make/keep it in that state

So 開けっぱなしにする means to leave it in the left-open state.

This is the same grammar as:

• きれいにする = make it clean
• 空っぽにする = empty it
• そのままにする = leave it as it is

Here, と means if or when.

In this sentence, it introduces a natural or automatic result:

• 窓を開けっぱなしにすると、カーテンがぬれてしまいます。
• If/when you leave the window open, the curtains get wet.

This と is often used when the result happens naturally, regularly, or inevitably.

So it fits well here because the speaker is describing a consequence: leave the window open on a rainy day → the curtains get wet.

This と is especially good for:

• natural consequences
• general truths
• habitual results
• things that happen automatically

That is why it sounds natural here.

Examples:

• 春になると、桜が咲きます。 = When spring comes, cherry blossoms bloom.
• このボタンを押すと、ドアが開きます。 = If you press this button, the door opens.

In your sentence, the result is also automatic and predictable, so と works very well.

A rough comparison:

• と = if/when X happens, Y naturally follows
• たら = if/when X happens, then...
• ば = if X, then Y
• なら = if it is the case that X / if you say X

Because they play different roles in the sentence.

• 窓を開けっぱなしにする
  窓 is the thing being left open, so it is the direct object of する in this expression. That is why it takes を.

• カーテンがぬれてしまいます
  カーテン is the thing that becomes wet, so it is the subject of ぬれる. That is why it takes が.

So:

• 窓を = the window is what you leave open
• カーテンが = the curtains are what get wet

ぬれる just means to get wet.

ぬれてしまう adds extra nuance. It often suggests:

• the action/result is completed
• it is unfortunate, unwanted, or regrettable

So:

• ぬれます = get wet
• ぬれてしまいます = end up getting wet / get wet unfortunately

In this sentence, the curtains getting wet is an undesirable result, so しまいます is very natural.

Because しまう attaches to the て-form of the verb.

The base verb is:

• ぬれる = to get wet

Its て-form is:

• ぬれて

Then add しまいます:

• ぬれてしまいます

This pattern works with many verbs:

• 忘れてしまいました = I ended up forgetting
• 食べてしまった = I ate it all / unfortunately ate it
• 壊れてしまいました = it broke / ended up broken

Yes, the subject is omitted.

Japanese often leaves out the subject when it is obvious, general, or not important. Here, the sentence is more like a general warning or statement:

On a rainy day, if you leave the window open, the curtains will get wet.

The hidden subject could be:

• you
• someone
• we
• people in general

Japanese does this very often, so not seeing a word for you is completely normal.

It can feel like either if or when, depending on context.

With と, the emphasis is on a regular or natural consequence. So the meaning is close to:

• If you leave the window open on a rainy day, the curtains will get wet.
• When you leave the window open on a rainy day, the curtains get wet.

English chooses between if and when, but Japanese と often sits somewhere between them when describing a dependable result.

Japanese often puts the setting first:

• time
• place
• topic
• condition

So 雨の日に comes first to establish the situation: on a rainy day.

Then comes the condition:

• 窓を開けっぱなしにすると

Then the result:

• カーテンがぬれてしまいます

This order is very natural in Japanese: setting → condition → result

It is very common and natural, and it often carries a slightly negative or careless nuance.

For example, 〜っぱなし can suggest:

• leaving something on
• not putting something back
• not finishing properly
• being careless

So 窓を開けっぱなしにする sounds a bit like leaving the window open carelessly rather than just neutrally saying the window is open.

That nuance matches the sentence well, because it leads to an undesirable result.

Not in this structure.

With 開ける, the thing you open normally takes を:

• 窓を開ける = open the window

Since 開けっぱなしにする is built from that idea, 窓を is the natural form.

Using 窓が would change the structure and would not fit this expression in the same way.

Yes. しまいます is the polite present/future form of しまう.

So the sentence is in polite style:

• ぬれてしまいます

A casual version would be:

• ぬれちゃう
• ぬれてしまう

Likewise, the whole sentence could become more casual:

• 雨の日に窓を開けっぱなしにすると、カーテンがぬれちゃう。

The original sentence is polite and very natural for explanation or advice.