Learn Hardcore Croatian: Djevojka je u školi.

Because the preposition u takes the locative case to express location (being in/at a place). The noun škola (school) in the locative singular is školi. So, for static location you say u školi.

When do I use u with the accusative instead (e.g., u školu)?

Use u + accusative to express motion into a place (direction). Compare:

Static location: Djevojka je u školi. = The girl is at/in school. (locative)
Motion/direction: Djevojka ide u školu. = The girl is going to school. (accusative)

What case is djevojka in, and why?

Djevojka is nominative singular (feminine) because it’s the subject of the sentence.

How do I pronounce this sentence?

Approximate English-friendly guide: DYE-voi-kah yeh oo SHKOH-lee.

j is like English “y” (as in “yes”)
š is like “sh”
u is like “oo” in “food”
The sequence dje is pronounced like “dye”

Why is it u školi and not na školi?

u is used for being “in/inside” or “at” most enclosed places and institutions, including schools: u školi.
na is used for surfaces and for some institutions/events: na poslu (at work), na fakultetu (at university), na koncertu (at a concert).
na školi would mean literally “on the school (building)” and is not used for the normal “at school” meaning.

How do I turn the sentence into a yes–no question: “Is the girl at school?”

Use the question particle li with the verb:

Je li djevojka u školi? You may also see the colloquial contraction Je l’ djevojka u školi?
Regionally (especially outside Croatia) you’ll hear Da li je djevojka u školi?, but in standard Croatian Je li is preferred.

How do I answer briefly “Yes, she is” or “No, she isn’t”?

Yes: Da. / Jest. (Formal/standard short form; standalone “je” is usually avoided in very careful speech.)
No: Ne. / Nije.

How do I negate the original sentence?

Replace je with nije:

Djevojka nije u školi. = The girl is not at school.

Can I drop je and just say Djevojka u školi?

Not in a normal, full sentence. Je (the verb “is”) is required. You might see Djevojka u školi in headlines, notes, or very telegraphic style, but it’s not a complete standard sentence.

What’s the difference between “at school” and “in school” meanings here?

U školi primarily means physically at/in the school.
If you want to stress attending school (as a student), you’d usually say:

ide u školu = goes to school (is a student)
pohađa školu = attends school

Are there articles like “the” or “a” in Croatian?

No. Croatian has no articles. Djevojka je u školi can mean either “The girl is at school” or “A girl is at school,” depending on context. You can add determiners for clarity:

ta djevojka = that girl
jedna djevojka = a/one (certain) girl

Does djevojka only mean “girl,” or can it mean “girlfriend”?

It can mean both, depending on context. In everyday speech, cura often means “girl” or “girlfriend.” For a little girl, use djevojčica. If you mean “my girlfriend,” say moja djevojka or moja cura.

What changes in the plural?

Subject: djevojke (girls; nominative plural)
Verb: su (are)
Sentence: Djevojke su u školi. = The girls are at school. If you mean “in the schools” (plural), it’s u školama (locative plural).

What are the other present-tense forms of biti (to be)?

ja sam (I am)
ti si (you are, singular)
on/ona/ono je (he/she/it is)
mi smo (we are)
vi ste (you are, plural or formal)
oni/one/ona su (they are, m/f/n)

Can I change the word order?

Yes. Word order is flexible and used to manage emphasis. Examples:

Neutral/new info at the end: Djevojka je u školi.
Emphasizing the location: U školi je djevojka. Note that je is a clitic and tends to appear in “second position” in the clause.

How would I ask “Where is the girl?”

Gdje je djevojka?
Notice je still sits after the first stressed element (Gdje).

Why do I sometimes see devojka instead of djevojka?

That’s the Serbian (Ekavian) form. Standard Croatian (Ijekavian) uses djevojka. Both mean “girl/young woman,” but the spelling reflects the standard of the language variety.

Any spelling tips for the special letters here?

š is a distinct letter (not “s”): škola/školi, not “skola/skoli.”
j is “y” (so je sounds like “yeh”).
The sequence dje is written with d + j + e and pronounced like “dye,” not with the single letter đ.