Learn Hardcore Japanese: hako no naka ni neko ga iru.

の links nouns. 箱の中 means “the inside of the box.”
に marks location for existence. With いる/ある, use に to say where something exists.
が marks the grammatical subject that newly appears/exists there: 猫が.

Why is いる used instead of ある?

Use いる for living/animate beings (people, animals, bugs). Use ある for inanimate things (books, trees, boxes). A cat is animate, so いる.

Can I use は instead of が after 猫? What changes?

猫がいる presents new information: “A cat exists (there).”
猫はいる makes “cat” the topic/contrast: “The cat is (there), at least,” or “As for the cat, it’s there.” It often implies contrast (maybe other animals aren’t). Both are grammatical; the nuance differs.

Can I use で instead of に for the location?

Not with existence verbs. With いる/ある, the location takes に. Use で when describing an action happening at a place:

Existence: 箱の中に猫がいる。
Action: 箱の中で猫が寝ている。 (A cat is sleeping in the box.)

What’s the polite form?

Use います:

箱の中に猫がいます。 Polite question: 箱の中に猫がいますか。

How do I make it negative?

Plain negative: 箱の中に猫はいない。 (Natural to use は before a negative.)
Polite negative: 箱の中に猫はいません。 You can say 猫がいない, but 猫はいない is more typical for “there aren’t any cats.”

Is the word order flexible?

Yes, both are natural:

箱の中に猫がいる。 (location first; common for existence)
猫が箱の中にいる。 (subject first; answers “Where is the cat?”) Word order shifts focus, not core meaning.

What’s the difference between 箱の中に猫がいる and 箱の中には猫がいる?

箱の中には topicalizes the location: “As for inside the box, there is a cat.” It can sound contrastive (e.g., not elsewhere) or emphasize that specific place.

Can I drop particles in casual speech?

Sometimes:

箱の中に猫いる？ (drop が, casual but common)
箱の中猫いる？ (dropping に is riskier; keep に for clarity)
Answer casually: 猫いるよ。 Avoid mixing dropped particles with very formal endings.

How do I read/pronounce the sentence?

Hiragana reading: はこのなかにねこがいる。 Romanization: Hako no naka ni neko ga iru.

Are spaces normal in Japanese writing?

No. They’re added here for learners. Normally you’d write: 箱の中に猫がいる。

How do I ask “Is there a cat in the box?”

Polite: 箱の中に猫がいますか。
Casual: 箱の中に猫いる？

How do I say “The cat is in the box” (a known/specific cat)?

Use topic は: 猫は箱の中にいる。 This assumes the listener knows which cat you mean.

How do I specify the number of cats?

Use the counter 匹（ひき）:

箱の中に猫が一匹いる。 (one cat)
箱の中に猫が二匹いる。 (two cats) You can also say 箱の中に一匹の猫がいる, but placing the counter after the noun is most common in speech.

Can I say 箱に猫がいる without 中?

It’s odd for “in the box.” 箱に is used for “at/on the box” in some contexts (e.g., 箱に名前が書いてある). To mean “inside,” say 箱の中に. For other positions:

On the box: 箱の上に
Under the box: 箱の下に
Next to the box: 箱の隣に

What does 中 read as here, and can it be read ちゅう?

Here it’s なか (“inside”). 中 can also be read ちゅう/じゅう in compounds like 授業中 (during class) or 一年中 (all year). With possession Xの中, read なか.

What’s the general template for existence statements?

New info: [Place] に [Thing] がいる/ある。
Known thing’s location: [Thing] は [Place] にいる/ある。 Example: 公園に犬がいる。/ 犬は公園にいる。

Can I use も to say “also”?

Yes:

箱の中にも猫がいる。 (There’s also a cat in the box.)
猫もいる。 (There’s a cat, too.) Particles stack like には or にも to add topic/also nuance.

How can I add a friendly or emphatic tone?

Sentence-final particles:

よ (assertive/new info): 箱の中に猫がいるよ。
ね (seeking agreement): 箱の中に猫がいるね。
よね (assert + confirm): 箱の中に猫がいるよね。

What are the honorific/humble forms of いる, and do I use them for cats?

Honorific: いらっしゃる
Humble: おる / おります Use them for people. For animals like cats, stick with いる/います unless you’re being playfully respectful.

Is いる ever written in kanji? What about 要る?

いる (to exist: animate) is usually written in kana, sometimes as 居る. 要る is a different verb meaning “to need.” Same pronunciation, different meaning and usage. Context distinguishes them.