Learn Hardcore Croatian: Trebam papir za zadaću.

trebam ends in -m, which marks 1st person singular, so it already means “I need.” You can add Ja trebam… for emphasis or contrast, but it’s not required.

What exactly is trebam, and is there another common way to say “I need”?

trebam = “I need,” 1st person singular present of trebati.
Very common alternative: Treba mi [nešto] = “I need [something]” (literally “[something] is needed to me”). Examples:
Trebam papir.
Treba mi papir. Both are natural in Croatian; the dative construction (Treba mi…) is extremely common in everyday speech.

Why is it za zadaću and not za zadaća?

Because the preposition za takes the accusative case when it means “for (the purpose of).”

zadaća (fem. nominative singular) → zadaću (accusative singular). So you get za zadaću = “for homework.”

Why doesn’t papir change form here?

papir is a masculine inanimate noun. For masculine inanimate nouns, the accusative equals the nominative in the singular:

nominative: papir
accusative: papir If it were plural, it would change: trebam papire (“I need papers/documents”).

Croatian has no articles. How do I show “a/some/the paper”?

Context usually covers this. If you need to be explicit:

“a/one”: jedan (e.g., trebam jedan list papira = “I need one sheet of paper”)
“some”: neki/nešto or use partitive genitive (see below)
“the”: taj/ta/to (“that”) or onaj/ona/ono (“that over there”) often serve as “the” in context. Often, plain papir is enough.

Does papir mean one sheet, or paper as a material? How do I say “a sheet/piece of paper”?

papir can mean the material in general or a sheet, depending on context. To be precise:

Can I say Treba mi papira or Trebam papira to mean “I need some paper (unspecified amount)”?

Yes—this is the “partitive genitive” for an indefinite quantity of a mass noun:

Very natural: Treba mi papira. (“I need some paper.”)
With the personal form, speakers usually say Trebam papir, but Trebam papira is heard in colloquial speech. Use Treba mi papira if you specifically want “some (amount of) paper.”

Is za zadaću enough, or should I say za domaću zadaću? Are there regional differences?

In Croatian, domaća zadaća is “homework.” In context, za zadaću is usually clear and natural.
If you want to be explicit: za domaću zadaću.
In Serbian/Bosnian you’ll often hear domaći zadatak. Croatians understand it, but the native Croatian term is domaća zadaća.

How flexible is word order? Can I say things differently for emphasis?

Word order is flexible and used for emphasis:

Neutral: Trebam papir za zadaću.
Emphasize purpose: Za zadaću trebam papir.
Emphasize the thing needed: Papir mi treba za zadaću. Note the clitic mi (if used) goes in the “second position”: Papir mi treba…, Za zadaću mi treba…

Should I say “for my homework” (za moju/svoju zadaću)?

Usually it’s understood from context, so you can omit “my.” If you want to include it:

With a 1st-person subject (as in Trebam papir…), use the reflexive possessive: za svoju zadaću (“for my own homework”).
With the impersonal form (Treba mi papir…), the grammatical subject is papir, so you must use moju (not svoju): Treba mi papir za moju zadaću.

What’s the difference between zadaća and zadatak?

zadaća (esp. domaća zadaća) = homework as a whole assignment.
zadatak = a task/problem/exercise (often one item within the homework).
So you’d more likely say papir za zadaću, not za zadatak, unless you mean “for that specific task.”

How do I pronounce the ć in zadaću, and why is it not č?

Croatian distinguishes “soft” ć and “hard” č:

č is like English “ch” in “church.”
ć is a softer, more delicate sound (somewhat between “ch” and “t”), shorter and lighter. The noun is zadaća, so the accusative is zadaću. Keep the same consonant: ć.

How do I say this in the past or future?

Two patterns (personal and dative/impersonal) behave differently:

Past:

Personal: Trebao sam papir za zadaću. (“I needed paper…”)
Dative: Trebao mi je papir za zadaću. (agrees with the grammatical subject: papir = masc.; e.g., Trebala mi je olovka for a fem. noun)

Future:

Personal: Trebat ću papir za zadaću.
Dative: Trebat će mi papir za zadaću. Note the placement of ću/će and mi (clitics) in second position.

How do I make it negative or ask a question?

Negative:
- Personal: Ne trebam papir za zadaću.
- Dative: Ne treba mi papir za zadaću.
Yes–no questions:
- Trebam li papir za zadaću?
- Treba li ti papir za zadaću? You can also use rising intonation: Trebaš papir za zadaću?