fuyu wa atsui kooto o kiru to atatakai desu.

In winter, it is warm if you wear a thick coat.

Breakdown of fuyu wa atsui kooto o kiru to atatakai desu.

• は: topic particle
• です: to be
• を: direct object particle
• 冬ふゆ: winter

Questions & Answers about fuyu wa atsui kooto o kiru to atatakai desu.

Why is 冬 followed by は instead of に?

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.

What does 厚いコート mean grammatically?

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.

Why is there an を after コート?

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.