Learn Hardcore Japanese: kyou ha amari atukunai desu.

は is the topic marker. It tells the listener “as for today.” In contrast, が marks a subject that’s new information or the focus of a sentence. Using 今日は sets up “today” as the context or theme of what follows.

How do you pronounce the particle は here?

When は functions as a topic particle, it’s pronounced wa, not ha. So 今日は is read きょう wa.

What is the function of あまり in this sentence?

あまり is an adverb meaning “not very” or “not much” when used with a negative verb or adjective. It downplays the degree of heat:
• あまり暑くない = “not very hot.”

Why do we say あまり暑くない instead of あまり暑い?

あまり requires a negative to express “not very.” If you say あまり暑い, it sounds incomplete or unnatural—listeners expect the negative. For “too hot,” you’d use とても暑い or すごく暑い, not あまり暑い.

How do you form the negative of an i-adjective like 暑い?

To make a plain (casual) negative of an i-adjective:
1) Drop the final い: 暑 → 暑
2) Add くない: 暑くない
So 暑い → 暑くない (“is not hot”).

What is the role of the final です in 暑くないです?

Adding です makes the sentence polite.
• 暑くない = casual negative (“is not hot”).
• 暑くないです = polite negative (“is not hot, sir/ma’am”).

Can we omit です at the end? What changes?

Yes. In informal or casual speech you can drop です:
• 今日(は)あまり暑くない。
This sounds more relaxed and conversational. Keeping です keeps it polite/formal.

What’s the difference between 暑くないです and 暑くありません?

Both are polite negatives, but:

暑くないです attaches です to the plain negative. It’s polite yet still close to casual speech.
暑くありません uses the formal negative of です itself. It sounds more formal or stiff.

Could we replace あまり with そんなに here?

Yes. そんなに暑くないです also means “It’s not that hot.” そんなに and あまり function similarly when paired with a negative, though そんなに often points to something just mentioned or implied.