Learn Hardcore Croatian: Voda je u hladnjaku; hladnjak je u kuhinji. - The water is in the fridge; the fridge is in the kitchen.

je is the 3rd person singular of biti (to be) and means is. In standard sentences you include it: Voda je u hladnjaku. Omitting it is mostly limited to headlines, notes, or very telegraphic style.

Why is it "u hladnjaku" and "u kuhinji"? Which case is that?

It’s the locative case, used with u to express location (being inside/in).

hladnjak (fridge, masculine) → u hladnjaku
kuhinja (kitchen, feminine) → u kuhinji

What’s the difference between "u hladnjaku" and "u hladnjak"?

u + locative = in/inside (static location): Voda je u hladnjaku.
u + accusative = into (movement toward): Stavljam vodu u hladnjak. Similarly: Hladnjak je u kuhinji. vs. Nosim hladnjak u kuhinju.

Why does one locative end in -u and the other in -i?

It depends on the noun’s gender/declension:

Most masculine nouns (ending in a consonant): locative singular -u → hladnjak → u hladnjaku, grad → u gradu.
Feminine -a nouns: locative singular -i → kuhinja → u kuhinji, voda → u vodi.
Neuter -o/-e nouns: usually -u/-ju → selo → u selu, polje → u polju.

Can I change the word order?

Yes. Common alternatives:

U hladnjaku je voda; u kuhinji je hladnjak.
U kuhinji je hladnjak; u hladnjaku je voda. Note: je is a clitic and tends to appear in the second position of the clause, so you cannot start with it (avoid: Je voda u hladnjaku).

Is the semicolon used the same way as in English?

Yes—here it links two closely related independent clauses. You could also write:

Voda je u hladnjaku. Hladnjak je u kuhinji.
Voda je u hladnjaku, a hladnjak je u kuhinji. (with a ~ “and/but” in contrastive sense)

Should I use "u" or "na" here? Could I say "na hladnjaku" or "na kuhinji"?

u = in/inside enclosed spaces: u hladnjaku, u kuhinji.
na = on top of surfaces or in set expressions: na hladnjaku means “on top of the fridge.” na kuhinji is not idiomatic; say u kuhinji (in the kitchen). For surfaces: na kuhinjskom stolu (on the kitchen table), na štednjaku (on the stove).

Is "hladnjak" the only word for fridge?

No. You’ll also hear frižider (very common in speech). Both are fine:

u hladnjaku / u frižideru Note: lednjak means “glacier,” not “fridge.”

How do I pronounce tricky parts like h, j, and nj?

h is a voiceless sound, like the German/Scottish ch in “Bach/loch”: (e.g., hladnjak, kuhinja).
j sounds like English y in “yes” (e.g., je, kuhinja).
nj is a single Croatian letter pronounced like Spanish ñ (e.g., kuhinja, hladnjak).
u is like the vowel in “food.”

How would I say it with plural subjects, e.g., “The bottles of water are in the fridge”?

Use the plural of biti:

Boce vode su u hladnjaku. (not je but su) For the second clause with a plural subject: Hladnjaci su u kuhinji.

Could I use "nalazi se" instead of "je"?

Yes. nalazi se = “is located/located itself,” a bit more formal or emphatic about location:

Voda se nalazi u hladnjaku.
Hladnjak se nalazi u kuhinji. The meaning is essentially the same, just a different nuance.

How do I say “from/out of the fridge/kitchen”?

Use iz + genitive for “out of/from (inside)”:

iz hladnjaka (from/out of the fridge)
iz kuhinje (from the kitchen)

How do I turn this into a yes–no question?

Use Je li…? (neutral/standard in Croatia):

Je li voda u hladnjaku?
Je li hladnjak u kuhinji? Colloquially you can also use rising intonation: Voda je u hladnjaku? You may also hear Da li…? regionally, but Je li…? is preferred in standard Croatian.