huyu ha atui kooto wo kiru to atatakai desu.

は marks 冬 as the topic: As for winter, ...

So 冬は厚いコートを着ると暖かいです has the feel of:

• As for winter, wearing a thick coat is warm
• or more naturally in English, In winter, if you wear a thick coat, you’ll be warm

If you used 冬に, that would mark winter more directly as a time when something happens:

• 冬に厚いコートを着る = wear a thick coat in winter

Both can work, but 冬は sounds like the speaker is talking generally about winter as a season.

厚い is an i-adjective, and it directly modifies the noun コート.

So:

• 厚い = thick
• コート = coat
• 厚いコート = a thick coat

In Japanese, adjectives normally come before the noun they describe, just like in English.

を marks the direct object of the verb.

Here, the verb is 着る (to wear), so コートを着る means:

• to wear a coat

The coat is the thing being worn, so it takes を.

Because と here is being used as a conditional meaning something like:

• if
• when
• whenever

With this use, the verb before と is usually in its plain form:

• 着ると = if/when [someone] wears

So:

• 厚いコートを着ると暖かいです = If you wear a thick coat, it’s warm / you’ll be warm

In this sentence, と is a conditional particle.

It often expresses a result that is:

• natural
• automatic
• generally true
• expected

So this sentence sounds like a general fact:

• If you wear a thick coat, you get warm
• Wearing a thick coat keeps you warm

This use of と is common when the second part is a natural consequence of the first.

暖かい is the predicate adjective here, meaning warm.

The structure is:

• 着ると暖かいです = if you wear it, it is warm / you will be warm

You would use 暖かく when connecting to another expression, for example:

• 暖かくなります = becomes warm
• 暖かくて気持ちいいです = it’s warm and feels nice

But here the sentence simply ends with the adjective, so 暖かいです is correct.

Because です here adds politeness, not new meaning.

• 暖かい = plain style
• 暖かいです = polite style

So the basic meaning stays the same:

• warm

Japanese often uses です after i-adjectives in polite speech.

Japanese often leaves out the subject when it is obvious or general.

Here, the sentence is giving a general statement, so the understood subject could be:

• you
• people
• one
• sometimes even I, depending on context

So 厚いコートを着ると暖かいです can mean:

• If you wear a thick coat, you’ll be warm
• When people wear thick coats, they stay warm

This omission is very normal in Japanese.

It sounds most natural as a general truth or general advice.

Because of 冬は and the conditional と, the sentence has the feeling of:

• In winter, wearing a thick coat keeps you warm
• If you wear a thick coat in winter, you’ll be warm

It does not sound like one specific event in the past or one single occasion.

Both are read あたたかい, but the kanji often suggest different kinds of warmth.

A common tendency is:

• 暖かい: warm weather, warm air, warm clothing, general warmth
• 温かい: something physically warm to the touch, like food, drinks, water, hands

Since this sentence is about staying warm by wearing a coat, 暖かい is a natural choice.

Yes, it is understandable and natural enough, especially for learners.

It expresses a general idea clearly:

• In winter, if you wear a thick coat, you’ll be warm

Depending on context, native speakers might also say things like:

• 冬は厚いコートを着ると暖かいですよ。
• 冬は厚いコートを着れば暖かいです。
• 冬は厚いコートを着ると暖かく過ごせます。

But the original sentence is perfectly good for learning the grammar.

Yes, and the nuance changes slightly.

• 着ると暖かいです = If/when you wear it, you’re warm
  This sounds like a natural result or general fact.

• 着れば暖かいです = If you wear it, you’ll be warm
  This can sound a little more like advice or a hypothetical condition.

Both are correct, but と is especially common for predictable results.

A good breakdown is:

• 冬は = as for winter
• 厚いコートを = a thick coat
  • object marker
• 着ると = if/when [someone] wears
• 暖かいです = it is warm / [you] are warm

So the overall pattern is:

Topic + object + verb + conditional + result

That is: 冬は｜厚いコートを｜着ると｜暖かいです。