aki ni naru to, asa no kuuki ga suzusikute kimotiii desu.

Because なる often takes に to mark the state something changes into.

• 秋になる = to become autumn / when autumn comes
• Here, に is not just a simple time marker. It shows the result of the change.

So 秋になると literally has the sense of when it becomes autumn.

Here, と is a conditional particle meaning when or whenever.

So:

• 秋になると、... = When autumn comes, ...
• It often suggests a natural, regular, or repeated result

That makes it a good fit for seasonal statements and general truths.

Compared with たら, と sounds more like:

• Whenever X happens, Y happens
• a usual pattern, not just one specific occasion

Because の connects two nouns.

• 朝 = morning
• 空気 = air
• 朝の空気 = the morning air

This is a very common Japanese pattern:

• 春の雨 = spring rain
• 日本の文化 = Japanese culture
• 学校の先生 = school teacher

So 朝の空気 is just the natural way to say morning air.

が marks 空気 as the subject of the description すずしくて気持ちいいです.

So the sentence is saying:

• the morning air is cool and pleasant

If you used は, it would sound more like you are setting morning air up as a topic, possibly with contrast:

• 朝の空気はすずしくて気持ちいいです。 = As for the morning air, it’s cool and pleasant.

That is also possible, but が is very natural when simply identifying what has those qualities.

Because すずしくて is the て-form of the い-adjective すずしい.

For い-adjectives, you change:

• 〜い → 〜くて

So:

• すずしい → すずしくて

This form is used to connect adjectives:

• すずしくて気持ちいい = cool and pleasant

It can mean:

• cool and pleasant, or
• because it’s cool, it feels nice

In this sentence, it mainly links the two descriptions smoothly.

No. They are different.

• すずしい / 涼しい = pleasantly cool
• 寒い = cold

So 涼しい has a positive, comfortable feeling, especially for weather, air, or a breeze. That is why it works well with 気持ちいい.

Both are used.

• 気持ちがいい is the more literal structure: the feeling is good
• 気持ちいい is a very common set expression meaning pleasant, feels good, or comfortable

In everyday Japanese, 気持ちいい is extremely natural. It behaves almost like a single adjective in many situations.

So this sentence sounds very normal.

In Japanese, when adjectives or verbs are linked together, usually only the final predicate carries the polite ending.

So:

• すずしくて = connective form
• 気持ちいいです = final polite predicate

This is standard Japanese grammar. You do not normally say something like すずしいですて.

Yes, from an English point of view, something is omitted.

Japanese often leaves out subjects that are obvious from context. It does not need a dummy subject like it in weather or seasonal statements.

So instead of saying something like:

• When it becomes autumn...

Japanese simply says:

• 秋になると...

Then the main subject of the second clause is clearly stated:

• 朝の空気が...

It sounds like a general seasonal pattern, not one single event.

That is largely because of 〜と:

• 秋になると suggests when autumn comes / whenever it becomes autumn

So the sentence means something like:

• this is what the morning air is like in autumn in general

If you wanted to talk about one specific occasion, a different structure such as たら or more context might be used.

Yes. A common kanji version is:

秋になると、朝の空気が涼しくて気持ちいいです。

A few notes:

• 秋 = autumn
• 朝 = morning
• 空気 = air
• 涼しい = cool, refreshing
• 気持ちいい is often written partly in kana, though 気持ち良い also exists

Writing すずしい in hiragana is also completely normal, especially in learner materials.