nekosuna ga hituyou desu.

It’s usually read ねこすな.

• 猫（ねこ） = cat
• 砂（すな） = sand

So 猫砂 literally means cat sand, but in natural English it means cat litter.

必要（ひつよう） means necessary or needed.

In this sentence, 猫砂が必要です means something like:

• Cat litter is needed.
• I need cat litter.
• Cat litter is necessary.

Which English translation sounds best depends on the context.

The particle が marks the thing that is needed.

So in:

• 猫砂が必要です

the cat litter is the thing being identified as necessary.

A very literal breakdown is:

• 猫砂 = cat litter
• が = marks what is needed
• 必要です = is necessary / is needed

So the structure is basically X が 必要です = X is needed.

In Japanese, 必要 is not a normal English-style verb like to need. It behaves more like a noun / な-adjective meaning necessary.

So Japanese often expresses this idea as:

• X が 必要です = X is necessary / needed

rather than using a direct verb equivalent of need the way English does.

That’s why the sentence feels more like Cat litter is necessary structurally, even though natural English may be I need cat litter.

です makes the sentence polite.

Since 必要 is a noun-like / な-adjective word, です works as the polite ending:

• 必要です = is necessary / is needed

Casual forms would be:

• 必要だ
• 必要

So 猫砂が必要です is the polite standard form.

It’s omitted because Japanese often leaves out information that is clear from context.

So 猫砂が必要です could mean:

• I need cat litter
• We need cat litter
• You need cat litter
• Cat litter is necessary

depending on the situation.

Japanese does this very often. If the speaker and situation are already understood, there is no need to say I explicitly.

Yes, you can say 猫砂は必要です, but the nuance changes.

• 猫砂が必要です: focuses on cat litter as the thing that is needed.
• 猫砂は必要です: makes cat litter the topic, often with a contrastive feeling, like as for cat litter, it is necessary.

So が is the more neutral choice when simply stating that cat litter is needed.

It’s similar, but not exactly the same in nuance.

You can also say:

• 猫砂がいる
• 猫砂が要ります

This uses the verb 要る（いる）, meaning to need.

The difference is roughly:

• 猫砂が必要です = cat litter is necessary / needed
  Slightly more formal, objective, or matter-of-fact.
• 猫砂が要ります = need cat litter
  More directly about needing it.

In everyday conversation, both can work, but 必要です often sounds a little more formal or explanatory.

Japanese usually does not mark articles like a / the and often does not mark plural unless it matters.

So 猫砂 can mean:

• cat litter
• some cat litter
• the cat litter

depending on context.

Japanese leaves that kind of detail unstated unless it is important.

The pattern is:

• Noun + が + 必要です

meaning:

• Noun is needed
• Need noun

Examples:

• 水が必要です。 = Water is needed.
• 時間が必要です。 = Time is needed.
• お金が必要です。 = Money is needed.

So 猫砂が必要です。 follows a very common and useful pattern.