ningen ha mizu ga hituyou desu.

Because they are doing different jobs.

• 人間は marks 人間 as the topic: as for humans...
• 水が marks 水 as the thing that is grammatically tied to 必要です: water is necessary

So the structure is roughly:

A は B が 必要です
= As for A, B is necessary

This is very common in Japanese. A sentence can have a topic marked by は and also another element marked by が inside the comment about that topic.

Because 必要です is not an action verb like drink or eat. It describes a state: being necessary.

In this pattern, the thing that is necessary is marked with が:

• 水が必要です = Water is necessary
• お金が必要です = Money is necessary
• 時間が必要です = Time is necessary

If you used を, it would sound wrong here because 必要です does not take a direct object the way an action verb does.

必要 is usually treated as a na-adjective or adjectival noun.

That means it can work like this:

• 水が必要です = Water is necessary
• 必要なもの = a necessary thing
• 必要ではありません = is not necessary

So in this sentence, 必要です means is necessary.

Yes. That is also natural, and in some contexts it is even more explicit.

• 人間は水が必要です = As for humans, water is necessary
• 人間には水が必要です = For humans, water is necessary / Humans need water

Here, に marks the person or thing for whom something is necessary, and は adds topic or contrast.

The shorter version without に is still very understandable, especially in a general statement like this.

Not exactly.

• 人 usually means person or people
• 人間 means human being(s) or humans

In a general statement about the human species, 人間 fits well. It sounds broader and more like human beings than 人 does.

So for a sentence like this, 人間 is a natural choice.

Yes, that is a good literal breakdown.

A very literal structure is:

• 人間は = as for humans
• 水が = water
• 必要です = is necessary

That literal wording sounds a little unnatural in English, but it helps show the Japanese grammar clearly.

Japanese usually does not mark articles or plurals the way English does.

So 人間 can mean:

• a human
• humans
• the human race

And 水 can mean:

• water
• the water

Context tells you which meaning is intended. In this sentence, it is understood as a general truth: Humans need water.

To some extent, yes, because particles show the roles of the words.

The neutral order here is:

人間は水が必要です

But other versions are possible, depending on what you want to emphasize. For example:

• 水は人間に必要です = As for water, it is necessary for humans
• 人間には水が必要です = Humans, at least, need water

So the basic meaning can stay similar, but the focus and nuance may change.

No. Normally, Japanese is written without spaces:

人間は水が必要です。

Spaces are often added in beginner materials to make the sentence easier to read.

It is read:

にんげん は みず が ひつよう です。

More naturally, without learner spacing:

にんげんはみずがひつようです。

Key readings:

• 人間 = にんげん
• 水 = みず
• 必要 = ひつよう

です makes the sentence polite.

So:

• 人間は水が必要です = polite
• 人間は水が必要だ = plain / less polite

Both mean basically the same thing, but です is the safer form for learners and is commonly used in normal polite conversation and writing.