Learn Hardcore Japanese: mise no mizu ha yuuryou da kara, kozeni ga nai to komarimasu. - Water at the store costs money, so it’s a problem if I don’t have coins.

の links two nouns: 店の水 = “the store’s water” / “water of the store.”
で would mean “at the store,” focusing on location: 店で水は有料だ = “At the store, water costs money.”
は would topicalize the store: 店は水が有料だ = “As for the store, the water is not free.”
Use の when you want a possessive/attributive relationship (“the store’s water”), and で/は when focusing on location/topic.

Why is は after 水? Could it be が instead?

水は marks water as the topic (“as for water”). It can also create a contrast (e.g., “as for water (unlike other things)…”).
水が would mark “water” as the subject/new info. 店の水が有料だ puts stronger emphasis on identifying water as the thing that’s charged.
In a general statement like this, 水は is natural.

What part of speech is 有料, and why do we need だ before から?

有料 is a noun (often treated like a na-adjective in behavior). Predicatively it takes the copula: 有料だ (“it’s fee-charging/not free”).
Before から with a noun/na-adjective, you need だ: 有料だから.
Alternatives:
Polite: 有料ですから
With ので: 有料なので (plain) / 有料ですので (polite)

Is it okay to mix だ (plain) with ます (polite) like 有料だ…困ります?

Yes. Japanese often sets politeness by the final verb. Subordinate clauses (before から/ので/と etc.) can be plain while the main clause is polite. For fully consistent politeness, you could say 有料ですから、困ります.

What’s the difference between から and ので here?

から: straightforward “because,” slightly more direct/casual.
ので: “since/as,” often softer/more objective or formal.
With nouns/na-adjectives: 有料だから vs 有料なので. In polite style, ですから / ですので are both common.

What exactly does 小銭 mean? How is it different from similar words?

小銭（こぜに）: small change/coins you pay with (especially low denominations).
硬貨（こうか）: coins in general (as opposed to bills).
お釣り（おつり） / 釣り銭（つりせん）: the “change” you receive back after overpaying.
Synonym for everyday speech: 細（こま）かいお金 (“small change”).

Why is it 小銭がない and not 小銭はない?

が marks existence/non-existence naturally: Xがない = “there isn’t X.”
小銭はない adds contrast or topic focus (“As for small change, there isn’t any…”), which can imply “but there is something else.” In neutral statements, がない is default.

What does the conditional と do in ないと困ります? Why not たら or ば?

と conditional expresses a natural, expected result: “If/when A happens, B happens (as a rule).”
Here: 小銭がないと困ります = “If I don’t have small change, I’ll be in trouble (that’s what happens).”
たら: more event-based/one-time (“if/when it turns out that…”).
ば: somewhat more hypothetical or formal.
All can work, but と fits the “whenever/if this condition holds, this problem results” nuance.

Could I say 小銭がなくて困ります instead?

Yes, but nuance changes:

小銭がないと困ります: conditional/result (“If/when I don’t have change, I’ll have trouble.”)
小銭がなくて困ります: direct cause (“I’m troubled because I don’t have change [now].”)
Use なくて for a current cause/complaint; use ないと for a condition leading to trouble.

Does ないと here mean “must,” like the shorthand for ないといけない?

Not in this sentence. ないと can be a contraction of “must” in patterns like 行かないと (gotta go), but here it’s clearly the conditional because it’s followed by a result: …と困ります.

Would 有料です be better than 有料だ?

For consistent politeness, yes:

Polite: 店の水は有料ですから、小銭がないと困ります。
Plain: 店の水は有料だから、小銭がないと困る。
Both are correct; choose based on context/audience.

Who is the implied subject of 困ります?

It’s omitted, as is common in Japanese. Context decides. Here it likely means “I” (speaker), “we” (our group), or even a general “you/people.” If you need to be explicit, you can add 私（わたし）は or お客様は, etc.

Is 店の水 natural? Would people say お水 or お冷（ひ）や?

In restaurants, people often say お水 (polite) or お冷や (the chilled tap water they serve).
店の水 is fine in a neutral, factual statement, especially if you mean “the water the store provides/sells.”
If you want a location/topic emphasis: この店では水は有料です。

Can I say 店では水は有料だ? Is double は okay?

Yes. 店では sets location/topic; 水は sets the specific item. この店では、水は有料です。 is perfectly natural.

Why is the particle は pronounced “wa” in 水は?

It’s a historical quirk: the topic particle is written は but pronounced “wa.” Similarly, the directional particle へ is pronounced “e,” and the object marker を is pronounced “o.”

What nuance does 困ります carry? Is it just “be in trouble”?

困る ranges from “to be in a bind/inconvenienced” to “that would be a problem.” …と困ります can also serve as a soft warning/request: “It would be a problem if … (so please …).” Here it’s “it’s a problem/inconvenient.”

Does 困ります refer to the future or present?

Japanese non-past covers both. Here it means “will be a problem” (future/conditional outcome) or a general rule (“it’s problematic when…”). Context decides.

Can I use the polite negative ありません: 小銭がありませんと困ります?

Grammatically possible but stiff/old-fashioned in speech. Natural options:

Mixed (very common): 小銭がないと困ります。
Fully polite but smooth: 小銭をお持ちでないと困ります。 / 小銭がございませんと困ります。 (very formal)

Do I need the comma before 小銭?

No. Commas in Japanese are flexible and used for readability. Both …だから小銭が… and …だから、小銭が… are fine.