natu no yoru ha ka ga ooi node, mado wo akeppanasi ni simasen.

の links two nouns, so 夏の夜 means summer night or nights in summer.

• 夏 = summer
• 夜 = night
• 夏の夜 = a summer night / summer nights

This is a very common pattern in Japanese:

• 日本の映画 = Japanese movie / movie from Japan
• 学校の先生 = school teacher

Here, 夏 is describing what kind of 夜 it is.

The は marks 夏の夜 as the topic of the sentence.

So 夏の夜は means something like:

• As for summer nights...
• On summer nights...

It sets the scene for what follows. The sentence is not just talking about night in general, but specifically summer nights.

In 蚊が多い, が marks 蚊 as the thing that is being described by 多い.

So:

• 蚊が多い = mosquitoes are numerous / there are many mosquitoes

This is a very common pattern:

• 人が多い = there are many people
• 木が多い = there are many trees

If you said 蚊は多い, it would sound more contrastive, like:

• Mosquitoes are many (as opposed to something else)

So が is the natural choice here.

多い is an i-adjective meaning many, numerous, or a lot.

In this sentence, 蚊が多い means:

• there are many mosquitoes
• mosquitoes are numerous

Japanese often uses 多い where English uses there are a lot of...

So even though English may sound more natural with there are many mosquitoes, Japanese expresses it as mosquitoes are many.

ので means because or since.

In this sentence:

• 蚊が多いので、窓を開けっぱなしにしません。
• Because there are many mosquitoes, I do not leave the window open.

Compared with から, ので usually sounds:

• a bit softer
• a bit more explanatory
• a bit more objective

So:

• 蚊が多いから = because there are many mosquitoes
• 蚊が多いので = because there are many mosquitoes

Both are correct, but ので often feels slightly more polite or matter-of-fact.

開けっぱなし means left open or left open without closing it.

It comes from:

• 開ける = to open
• っぱなし = leaving something in that state, often continuously or carelessly

So:

• 窓を開けっぱなしにする = to leave the window open

This is different from just 窓を開ける, which only means to open the window.

Important nuance: っぱなし can sometimes suggest that something is left that way without taking care of it properly, like:

• ドアを開けっぱなしにする = leave the door open
• 電気をつけっぱなしにする = leave the light on

This is because of the pattern:

• X にする = to make/leave something in state X

So:

• 開けっぱなしにする = to leave it open
• literally: to make it into the state of being left open

In the sentence:

• 窓を開けっぱなしにしません
• I do not leave the window open

This is a very useful pattern:

• 静かにする = make it quiet / keep quiet
• きれいにする = make it clean
• そのままにする = leave it as it is

Because the action is not simply opening the window. The full expression is:

• 窓を開けっぱなしにする = to leave the window open

So the negative becomes:

• 窓を開けっぱなしにしません = do not leave the window open

If you said:

• 窓を開けません that would mean:
• I do not open the window

That is a different meaning.

So しません is negating the whole expression 開けっぱなしにする, not just the verb 開ける.

窓 is the direct object of the action.

The full action is:

• 窓を開けっぱなしにする
• to leave the window open

So を marks 窓 as the thing affected by the action.

Compare:

• ドアを閉める = close the door
• テレビをつけっぱなしにする = leave the TV on
• 本を机の上に置きっぱなしにする = leave the book sitting on the desk

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

So しません could mean:

• I do not
• we do not
• he/she/they do not

depending on the situation.

In a normal standalone sentence like this, learners usually understand it as:

• I don't leave the window open or
• we don't leave the window open

Japanese does this very often. You do not need to state the subject if it is already clear.

It can be either. Japanese nouns usually do not show singular/plural the way English does.

So 夏の夜は could mean:

• on a summer night
• on summer nights
• as for summer nights

In this sentence, the meaning is probably summer nights in general, so English often translates it as on summer nights.

Yes, it often does.

っぱなし can suggest:

• something is left that way continuously
• someone did not bother to change it
• a somewhat careless state

So 窓を開けっぱなし is not just open. It is more like:

• left open
• left wide open
• kept open and not closed

That nuance is why this expression is so common in everyday Japanese.

It would still be correct:

• 夏の夜は蚊が多いから、窓を開けっぱなしにしません。

This version sounds a bit more direct and conversational.

Very roughly:

• ので = slightly softer, more explanatory
• から = slightly more direct, personal

Both are natural, but the original with ので has a calm, explanatory feel.