Learn Hardcore Japanese: syukudai wo sinai to, syukudai ga tamarimasu. - If you don't do your homework, your homework piles up.

In 宿題をしないと, 宿題 is the object of the verb する (to do).
- を marks the direct object:
  - 宿題をする = to do homework
  - 宿題をしない = to not do homework
In 宿題がたまります, 宿題 is the subject of the verb たまる (to pile up / accumulate).
- が marks the subject:
  - 宿題がたまる = homework piles up / homework accumulates

So literally:
(If) you don’t do homework, homework piles up.

Same noun, two roles: object in the first clause, subject in the second, so two different particles.

What does と mean here? I thought と meant “and” or was used for quotes.

Here, と is not “and” or the quotation particle. It’s the conditional と, which often corresponds to “when / if (X happens), (Y) automatically follows”.

Pattern: [plain form] + と、[result]
Sense: whenever X happens, Y happens as a natural or predictable result.

In this sentence:

宿題をしないと = if you don’t do your homework / when you don’t do your homework
宿題がたまります = the homework piles up

So it means:

If you don’t do your homework, the homework piles up.
Or: When you don’t do your homework, it tends to pile up.

Key nuance: と-conditionals are often used for fairly automatic or natural consequences.

Why is it しない and not しません? Is the sentence casual or polite?

しない is the plain (informal) negative form of する.
しません is the polite negative form.

する → しない (plain)
する → しません (polite)

The sentence as written:

宿題をしないと、宿題がたまります。
→ mixed style: first clause is plain (しない), second clause is polite (たまります).

In natural usage, you would typically match the levels:

Casual:
- 宿題をしないと、宿題がたまる。
Polite:
- 宿題をしないと、宿題がたまります。
  (This is actually common; people often use plain in the conditional clause even in polite speech.)
Fully consistent polite version (more textbook-like):
- 宿題をしませんと、宿題がたまります。 (grammatical but sounds stiff / old-fashioned in everyday speech)

So: the sentence is fine for everyday polite speech; using しない in the conditional clause with たまります is normal in conversation.

Could you also say 宿題をしなかったら、宿題がたまります? What’s the difference from しないと?

Yes, you can say:

宿題をしなかったら、宿題がたまります。

Both 〜ないと and 〜なかったら can express “if you don’t…”.

Nuances:

〜ないと with a comma and a clear result clause:
- 宿題をしないと、宿題がたまります。
  → “If you don’t do your homework, the homework piles up (as a consequence).”
  Slightly more general / habitual feel, especially with と.
〜なかったら:
- 宿題をしなかったら、宿題がたまります。
  → Also “if you don’t do your homework, the homework piles up.”
  〜たら tends to feel a bit more like a specific situation or condition and is very commonly used in conversation.

In this particular sentence, the difference is subtle and both are natural. 〜ないと with と emphasizes the “whenever/if X then naturally Y” relationship a bit more.

I’ve also seen 〜ないと used as “I have to ~.” Is that what’s happening here?

Not in this sentence. 〜ないと has two common uses:

Conditional “if (you) don’t ~, then…”
- Structure: [negative plain form] + と、[result]
- Example:
  - 宿題をしないと、宿題がたまります。
    “If you don’t do your homework, it piles up.”
Colloquial “I have to ~ / You gotta ~.”
(Short for 〜ないといけない, 〜ないとダメ, etc.)
- Example:
  - 宿題をしないと。 (spoken)
    → (implied) 宿題をしないといけない。
    → “I have to do my homework.”

In your sentence, there is a comma and a clear second clause (宿題がたまります), so it’s clearly the conditional use, not the “I have to” use.

What exactly does たまります mean? Is it just “to increase”?

たまります is the polite present form of たまる.

Core meaning here: to accumulate / to pile up / to build up.

宿題がたまる
→ Homework accumulates / Homework piles up.

Other examples:

仕事がたまっています。
→ Work is piled up.
洗濯物がたまった。
→ The laundry has piled up.

There is also a different usage of たまらない meaning “unbearable / can’t stand it” (e.g., うるさくてたまらない = “so noisy I can’t stand it”), but that’s from the same verb in a different construction and meaning. In your sentence, it’s simply “to pile up.”

Why is 宿題 repeated? In English we might say “If you don’t do your homework, it piles up.” Couldn’t Japanese just say 宿題をしないと、たまります?

Repeating 宿題 is very natural in Japanese to keep things clear:

宿題をしないと、宿題がたまります。

You can say:

宿題をしないと、たまります。

and people will understand from context that it = homework. But:

In Japanese, pronouns like “it” are used less than in English.
Japanese often either repeats the noun or omits it completely when obvious.

Other natural variants:

宿題をしないと、どんどんたまりますよ。
(no subject stated; understood as “it” = homework)
宿題をしないと、それがたまります。
(you could say それ, but it sounds a bit stiff or overly explicit here)

So the repetition of 宿題 is normal and clear; it’s not considered bad style.

What is the subject of たまります? Is it me? The homework? Something else?

The subject is 宿題 in the phrase 宿題がたまります.

が marks the subject or focus:
- 宿題がたまります。
  → Homework piles up.

The subject is not “I” or “you” here. Instead, homework itself is described as the thing that accumulates.

Literally:

“If you don’t do homework, homework accumulates.”

Japanese often describes things happening to the homework, work, laundry, etc., rather than using English-like “I have a lot of homework.”

Can the word order be changed, like 宿題がたまります。宿題をしないと。?

In natural Japanese:

It’s very typical for the condition to come before the result.
- 宿題をしないと、宿題がたまります。 (natural)

Your suggested order:

宿題がたまります。宿題をしないと。

is not very natural as a single coherent explanation. It might appear in dialogue as two separate thoughts, e.g.:

A: 最近どう？ (How’s it going?)
B: 宿題がたまります。宿題をしないと。
(Sounds broken/odd; you’d more likely say something like
宿題をしないから、宿題がたまります。)

As a rule of thumb for conditionals:

Use [condition]、[result]:
- 宿題をしないと、宿題がたまります。

Why is the verb in the second part たまります (present polite) instead of past or future? It sounds like present but the meaning feels like future.

Japanese non-past form (〜ます / 〜る) covers both present and future (and even general truths/habits).

So:

宿題がたまります。
- Can mean:
  - Homework piles up (habitually / generally).
  - Homework will pile up (future result).

In a conditional like:

宿題をしないと、宿題がたまります。

the time is understood from context:

“If you don’t do your homework (now / in general), it will pile up.”

Japanese doesn’t need a separate explicit “will”; たまります already covers that sense as part of the conditional structure.

Is there a more formal or textbook-like version of this sentence?

Yes. Several more formal/“textbook” versions are possible. For example:

Using 〜なければ:
- 宿題をしなければ、宿題がたまります。
  → “If you do not do your homework, your homework will accumulate.”
Using 〜ないといけません to stress obligation:
- 宿題をしないと、宿題がたまってしまいます。
  (still fairly natural, semi-formal)
- Or: 宿題をしないといけません。しないと、宿題がたまってしまいます。

But your original:

宿題をしないと、宿題がたまります。

is already perfectly good, natural Japanese, especially in explanations or everyday polite speech.