atui hi ha mizu wo yoku nomimasu.

It is read:

あつい ひ は みず を よく のみます。

A few helpful points:

• 暑い日 = あつい ひ
• は is the topic particle, and here it is pronounced わ
• 水 = みず
• 飲みます = のみます

In normal Japanese writing, spaces are usually not used, so you would normally see:

暑い日は水をよく飲みます。

In Japanese, words that modify a noun usually come before the noun.

So:

• 暑い日 = hot day
• literally, hot + day

This is the normal pattern for adjectives in Japanese. English does the same thing here, so this part is actually quite straightforward.

Both can relate to hot, but they are used differently.

• 暑い is used for weather, air temperature, or a hot day
• 熱い is used for things that are physically hot to the touch, like soup, coffee, or a pan

Since this sentence is talking about a day, 暑い is the correct word.

When は is used as a particle, it is pronounced わ.

So in this sentence:

• 日は is read ひ は in writing
• but pronounced ひ わ

This is a special feature of a few Japanese particles. The spelling stays traditional, but the pronunciation is different in modern Japanese.

暑い日は means something like:

• on hot days
• as for hot days
• when it is hot

The particle は marks 暑い日 as the topic. It sets the scene for the rest of the sentence.

So the sentence is not just saying hot day as a plain noun phrase. It is saying, roughly:

As for hot days, ...

That is a very common question.

Time expressions in Japanese do not always need に. In this sentence, 暑い日 is being used more like a topic/time setting, and は is doing that job.

So:

• 暑い日は = on hot days / as for hot days

You do not need に here.

Using に would sound unnatural in this sentence.

The particle を marks the direct object of the verb.

Here, the verb is 飲みます = drink

So 水を means:

• water as the thing being drunk
• literally, water + object marker

In other words, を answers the question what do you drink?
Answer: 水

Here, よく means often or frequently.

So 水をよく飲みます means:

• drink water often
• depending on context, it can also feel like drink plenty of water

Be careful: よく can also mean well in other sentences. For example:

• 日本語をよく話します could mean speaks Japanese well

But in this sentence, with drinking water on hot days, the natural meaning is often.

It is omitted.

Japanese often leaves out the subject when it is already understood from context.

So this sentence could mean:

• I drink water often on hot days
• You drink water often on hot days
• People drink water often on hot days

In many textbook examples, the implied subject is often I, but the actual subject depends on the situation.

飲みます is the polite non-past form of 飲む.

A few things to know:

• 飲む = plain form, casual
• 飲みます = polite form

The non-past in Japanese can express:

• a present habit
• a general truth
• a future action, depending on context

In this sentence, it expresses a habitual action:

• On hot days, I often drink water

So it is not talking about just one specific hot day. It describes what someone generally does.