Learn Hardcore Japanese: kyou ha atui desu kara, mizu wo nomimasu.

は is the topic marker. By saying 今日は, you set “today” as the topic of the sentence (as for today…). In English we might translate that as “Today, …” before stating what’s happening.

What’s the role of です after 暑い?

です is the polite copula. Although 暑い is an i-adjective and can stand alone, adding です makes the statement more polite.

Plain: 暑い (It’s hot.)
Polite: 暑いです (It’s hot, sir/ma’am.)

What function does から serve in this sentence, and how do you attach it?

から means “because,” linking a reason clause to the main clause. You can attach it to:

The plain form of an i-adjective: 暑いから、…
The polite form with です: 暑いですから、…
Both convey “Because it’s hot, …” with the second sounding more polite.

Could I connect these two ideas in a different way?

Yes. Common alternatives include:

Using ので for a slightly softer/neutral reason: 暑いので、水を飲みます。
Using the て-form of the adjective to link: 暑くて、水を飲みます。
• ～ので is more formal and often more explanatory.
• ～くて simply joins two states/events and can feel more narrative.

Why is the subject (like “I”) missing in this sentence?

Japanese often omits subjects when they’re clear from context. Here, drinking water is assumed to be what I (the speaker) will do. If you needed to specify it, you could say 私は水を飲みます.

Why is を used between 水 and 飲みます?

を marks the direct object of the verb. It tells you what is being acted upon. In this case, 水を飲みます means “(I) drink water,” with 水 being the thing you drink.

What’s the difference between 暑い and 熱い, and why is 暑い used here?

Both can mean “hot,” but:

暑い describes weather or ambient temperature (e.g. days, seasons).
熱い describes the heat of objects you can touch (e.g. tea, metal).
Since we’re talking about the weather (“Today is hot”), we use 暑い.