Learn Hardcore Japanese: fuyu wa samui desu.

は is the topic marker. It tells the listener “as for winter…” before making a comment about it. In English you might say “Speaking of winter, it’s cold.” Grammatically, は sets up 冬 as the theme of the sentence rather than marking it as the grammatical subject.

Why is は pronounced “wa” here and not “ha”?

Japanese retains historical spellings for certain particles. Although you write the topic marker as は, it’s always pronounced wa when used as a particle. The same goes for へ (written “he” but pronounced e when marking direction).

What part of speech is 寒い, and how does it function in this sentence?

寒い (samui) is an i-adjective (い形容詞). In Japanese, i-adjectives can:

Directly end a sentence and express a state (“it is cold”).
Conjugate to negative/past forms without a separate verb (e.g. 寒くない, 寒かった).

Why isn’t there a separate verb like “to be” in this sentence?

In English you need “is,” but in Japanese an i-adjective itself carries the meaning “to be [adjective].” Saying 寒い already means “it is cold.” No extra verb is needed.

What role does です play at the end of 冬は寒いです?

です is a polite sentence ending (often called the copula in beginner texts). When you attach です to an i-adjective, it doesn’t change the basic meaning— it simply raises the politeness level. Without です, you’re speaking in plain form; with です, you’re in polite form.

Can I drop です and just say 冬は寒い?

Yes. 冬は寒い is the plain/dictionary form and is perfectly natural in casual situations (talking with friends, family, etc.). Use 冬は寒いです when you need to be polite.

How do I make this sentence negative or put it in the past tense?

Because 寒い is an i-adjective, you modify its ending before adding です (if you want to stay polite):

Negative polite: 冬は寒くないです (It is not cold in winter.)
Negative plain: 冬は寒くない
Past polite: 冬は寒かったです (It was cold in winter.)
Past plain: 冬は寒かった

Could I say 冬が寒いです instead of 冬は寒いです? What’s the difference?

Grammatically, you can swap は and が, but the nuance changes. 冬が寒いです tends to emphasize that “winter” is the one that’s cold (perhaps contrasting with other seasons). For a simple statement like “winter is cold,” native speakers almost always use 冬は寒いです (topic-comment style).