mado wo akeru to, kimotiii kaze ga hairimasu.

It has two main parts:

• 窓を開けると = when/if you open the window
• 気持ちいい風が入ります = a pleasant breeze comes in

So the pattern is:

• [verb-dictionary form] + と, [result]

This often shows a result that naturally happens after the first action.

Because 開ける is the normal transitive verb here: to open something.

• 窓を開ける = open the window
• 窓が開く = the window opens

So:

• 開ける takes an object with を
• 開く is usually used when the thing opens by itself or is simply in the open state

That is why 窓を開ける is used in this sentence.

Here, と means when or if in the sense of a natural, automatic result.

So 窓を開けると、気持ちいい風が入ります means something like:

• When you open the window, a pleasant breeze comes in
• If you open the window, a pleasant breeze comes in

The nuance of と is often that the second part happens as a regular or expected consequence of the first part.

Because 窓 is the direct object of 開ける.

• 窓を開ける = open the window

The particle を marks the thing that the action is done to.

Compare:

• 窓を開ける = someone opens the window
• 窓が開く = the window opens

Because 風 is the subject of 入ります.

• 風が入る = the wind/breeze comes in

The breeze is the thing doing the action of entering, so が is used.

If you used を, you would usually need a different verb, such as 入れる:

• 風を入れる = let some air in / air out the room

But that is not the structure used here.

It is the polite form of 入る, an intransitive verb meaning to enter or to come in.

With 風, natural English translations are:

• a breeze comes in
• air comes in
• a pleasant breeze flows in

So even though 入る literally means enter, in English comes in sounds more natural here.

Because Japanese modifiers come before the noun they describe.

So:

• 気持ちいい風 = pleasant breeze / nice-feeling breeze

This is the same basic idea as:

• 大きい家 = big house
• 新しい本 = new book

Here 気持ちいい describes what kind of breeze it is.

No. A learner may also see 気持ちのいい風.

Both are used, and both mean a pleasant, comfortable breeze.

Very roughly:

• 気持ちいい風 feels a bit more direct and conversational
• 気持ちのいい風 can sound a little more formal or descriptive

In this sentence, 気持ちいい風 is perfectly natural.

Japanese often leaves out the subject when it is obvious from context.

So 窓を開けると could mean:

• when you open the window
• when I open the window
• when we open the window

The sentence does not explicitly say. English usually forces you to choose a subject, but Japanese often does not.

Because the Japanese non-past form covers several ideas, including:

• present
• future
• habitual/general truth

Here it expresses a regular result:

• When the window is opened, a pleasant breeze comes in

So 入ります does not mean only will enter. It can also mean comes in as a general fact.

Yes, but the nuance changes.

• 開けると emphasizes a natural or automatic result
• 開けたら is more flexible and can mean when/after if
• 開ければ is a more straightforward conditional if

In this sentence, と is a very natural choice because the second part is the kind of thing that normally happens as a result of the first.

Because the final verb is 入ります, the ます-form of 入る.

• 入る = plain form
• 入ります = polite form

So this sentence is suitable for normal polite conversation or textbook-style Japanese.

A casual version would be:

• 窓を開けると、気持ちいい風が入る。