Learn Hardcore Croatian: Kupaonica je čista, ali ključ je kod kuće. - The bathroom is clean, but the key is at home.

Because adjectives agree with the subject’s gender and number. Kupaonica is feminine singular, so the predicate adjective is feminine singular nominative: čista.

Masculine singular: čist (e.g., ključ je čist)
Feminine singular: čista (e.g., kupaonica je čista)
Neuter singular: čisto (e.g., sapun je čisto is wrong; it would be sapun je čist because sapun is masculine; neuter example: mlijeko je čisto)

What case is kuće in kod kuće, and why?

Kuće is genitive singular of kuća. The preposition kod governs the genitive and means “at/at the place of.” The fixed phrase kod kuće means “at home.” Don’t say kod kući (wrong case).

What’s the difference between kod kuće, u kući, and kući?

kod kuće = at home (idiomatic, default for “at home”)
u kući = in/inside the house (physical interior of the building)
kući (no preposition) = homeward/to home (with motion verbs: Idem kući = I’m going home) Saying X je kući for “X is at home” occurs colloquially in some regions, but standard Croatian prefers X je kod kuće.

Why is je in second position, and what happens after ali?

Je (is) is a clitic and normally sits in second position of its clause (after the first stressed word/phrase): Kupaonica je čista; Ključ je kod kuće.
After the conjunction ali, start a new clause and still place the clitic second there: … ali ključ je kod kuće. Placing it right after ali (… ali je ključ …) is generally avoided in standard style.

Do I need a comma before ali?

Yes. Croatian uses a comma before adversative conjunctions like ali.
Correct: Kupaonica je čista, ali ključ je kod kuće.

What’s the difference between ali, a, and no?

ali = but (neutral “but,” introduces a clear contrast)
a = and/while/whereas (milder contrast or simple juxtaposition)
no = but/however (often more formal/literary, strong adversative like “however”) All take a comma before them when connecting clauses.

Can I omit je in the present tense?

No. In standard Croatian the present of biti (to be) is used: je, su, etc.
Negative: nije (is not).
You can also use emphatic jest to carry stress: Kupaonica jest čista, ali… (indeed is).

Why are there no words for “the”?

Croatian has no articles. Definiteness (“the” vs. “a”) is inferred from context, word order, or by using demonstratives: ta kupaonica (that bathroom), ova kupaonica (this bathroom).

What genders are kupaonica and ključ, and why does it matter?

kupaonica = feminine; thus čista
ključ = masculine; if you described it, you’d say ključ je čist Adjectives and some pronouns must agree in gender and number with the noun.

How would this look in the plural?

Kupaonice su čiste, ali ključevi su kod kuće.
Notes:
Plural of kupaonica → kupaonice
Plural of ključ → ključevi
3rd person plural of biti is clitic su (second position).

How do I pronounce and tell apart č and ć, and what about lj in ključ?

č: hard “ch,” like English “ch” in “church” (e.g., čista, ključ)
ć: a softer, more delicate sound, somewhat like “t” + “y” blended; shorter and sharper (e.g., kuće)
lj: a single sound [ʎ], similar to the “lli” in English “million” when said quickly (in ključ) Writing without diacritics (e.g., kuca vs. kuća) can change meanings.

Is kupaonica the only way to say “bathroom”?

Standard Croatian: kupaonica (also common colloquial kupaona)
WC / toalet: toilet/restroom (often specifically the toilet)
kupatilo: common in Serbian/Bosnian; heard in parts of Croatia but not the standard Croatian choice.

Can I say Kupaonica je čisto?

No. Čisto is neuter or an adverb (“cleanly,” “quite”), not a feminine predicate adjective. You need feminine čista.
Note: čisto can mean “quite/rather” in some contexts (e.g., čisto dobro = quite good), but not here.

How do I turn this into a question or a negation?

Yes/no question: Je li kupaonica čista?; Je li ključ kod kuće?
Negation: Kupaonica nije čista; Ključ nije kod kuće.
You can also use intonation for questions in speech.

How do I specify whose home the key is at?

Use kod + Genitive (person) or a pronoun:

ključ je kod mene/tebe/njega/nje/nas/vas/njih = at my/your/his/her/our/your/their place
With a name: ključ je kod Marka.
If you specifically want “at my home,” you can also say kod mene doma (colloquial) or keep the neutral kod kuće and add context.