Learn Hardcore Croatian: Ovaj vikend imamo mali turnir u košarci i odbojci. - This weekend we have a small tournament in basketball and volleyball.

Both are grammatical; ovaj vikend is a bit more colloquial and direct, while ovog vikenda can sound slightly more formal or explicit about the time span.
ovo vikend is wrong because ovo is the neuter form, and vikend is masculine.

Why is there no word for “we”? Why does it just say imamo and not mi imamo?

Croatian is a pro‑drop language, which means subject pronouns (like ja, ti, on, mi) are usually omitted when the verb ending already shows who the subject is.

imamo = we have (1st person plural is clear from the ending ‑mo)

So:

Ovaj vikend imamo mali turnir…
= This weekend we have a small tournament…

Adding mi is possible:

Mi ovaj vikend imamo mali turnir…

…but that usually adds emphasis:

Mi ovaj vikend imamo mali turnir (a ne oni).
We have a small tournament this weekend (not them).

What is the literal word order, and could I move parts of the sentence around?

Literal order:

Ovaj vikend – This weekend (time expression)
imamo – we have
mali turnir – a small tournament (direct object)
u košarci i odbojci – in basketball and volleyball (prepositional phrase)

Croatian word order is quite flexible, especially for known information. These are all correct with small shifts in emphasis:

Imamo ovaj vikend mali turnir u košarci i odbojci.
Ovaj vikend mali turnir imamo u košarci i odbojci.
Mali turnir u košarci i odbojci imamo ovaj vikend.

The original sentence is neutral and natural; changing the order usually highlights whatever you move toward the beginning.

Why is it mali turnir and not malo turnir or something else?

Turnir is masculine singular. The adjective mali (small) must agree with it:

mali turnir = small tournament (masc. sg. nom./acc.)

Forms of mali in singular:

Masculine: mali (nom./acc. for inanimate)
Feminine: mala
Neuter: malo

So:

mali turnir (masc.)
mala utakmica (fem. – a small match)
malo selo (neut. – a small village)

Here, mali turnir is in the accusative (direct object of imamo), but for inanimate masculine nouns, nominative and accusative look the same.

Why is it u košarci i odbojci, not u košarku i odbojku?

The preposition u can take locative or accusative, depending on meaning:

u + locative: location, field, activity
- turnir u košarci = tournament in basketball (as a sport)
u + accusative: movement into something
- ići u kuću = to go into the house

In u košarci i odbojci, we’re talking about the type of tournament, i.e., a tournament in the sport of basketball and volleyball, so we use locative singular:

košarka → u košarci
odbojka → u odbojci

u košarku / u odbojku would usually imply movement into those things, which doesn’t fit here.

Why is u written only once: u košarci i odbojci, not u košarci i u odbojci?

In Croatian, if the same preposition applies to multiple nouns, it is common and natural to use it only once:

u košarci i odbojci
s prijateljima i kolegama (with friends and colleagues)
na stolu i stolici (on the table and the chair)

You can repeat u:

u košarci i u odbojci

This is still correct and may sound a bit more careful/emphatic, but the shorter version is perfectly standard.

What case are košarci and odbojci, and how do they change from the base form?

Both košarci and odbojci are in the locative singular (after u, with the “in the field of / in the area of” meaning).

Base forms (nominative):

košarka (basketball)
odbojka (volleyball)

Locative singular for many feminine nouns ending in ‑ka:

košarka → košarci
odbojka → odbojci

So the pattern (for these words) is roughly ‑ka → ‑ci in the locative singular.

Could I say turnir iz košarke i odbojke instead of turnir u košarci i odbojci?

You can, but there’s a nuance:

turnir u košarci i odbojci – the usual, neutral way: a tournament in basketball and volleyball (the sports themselves).
turnir iz košarke often appears in school contexts: a test/competition in the subject of basketball, or it can sound more like “from the area of”.

For sports events, turnir u [sport]u is more idiomatic:

turnir u nogometu – football/soccer tournament
turnir u tenisu – tennis tournament

So in this sentence, u košarci i odbojci is the most natural choice.

Is imamo here more like English “we have” or “we are having”?

Croatian doesn’t make a strict grammatical distinction between simple and continuous present the way English does.

Imamo mali turnir can be translated as either:
- We have a small tournament (this weekend).
- We are having a small tournament (this weekend).

Context decides how you render it.
Here, with Ovaj vikend, English usually prefers “we’re having”, but Croatian just uses present (imamo) for both.

Could I say ovaj vikend ćemo imati mali turnir instead? What’s the difference?

Yes:

Ovaj vikend imamo mali turnir.
Ovaj vikend ćemo imati mali turnir.

Both can describe a planned future event.

Nuance:

imamo (present) with a future time expression is very common and sounds like a scheduled/arranged event (similar to English We have a tournament this weekend).
ćemo imati (future) focuses more explicitly on the future aspect: we will have a tournament.

In everyday speech about fixed plans (classes, events, meetings), the present (imamo) is extremely common.

Can ovaj vikend appear at the end of the sentence, like in English?

Yes:

Imamo mali turnir u košarci i odbojci ovaj vikend.

This is totally natural.
Croatian time expressions (like ovaj vikend, sutra, danas) can appear:

at the beginning: Ovaj vikend imamo…
after the verb: Imamo ovaj vikend…
at the end: …ovaj vikend.

Position can slightly change the focus, but all are grammatically fine.

Is vikend a Croatian or a borrowed word, and does it decline like normal nouns?

Vikend is a loanword from English (weekend), but in Croatian it behaves like a regular masculine noun and is fully declined.

Singular (common forms):

Nominative: vikend – Ovaj vikend je slobodan.
Accusative: vikend – Ovaj vikend imamo turnir.
Genitive: vikenda – Nema vikenda bez utakmice.

So yes, it declines as a standard masculine noun, even though it’s borrowed.

How do I pronounce košarci and odbojci?

Approximate English-based guidance:

košarci:
- ko like ko in Kobe (but shorter)
- šar like shar in sharp (without the p)
- ci like tsi in cats
  - ee
    → roughly: koh‑SHAR‑tsee
odbojci:
- od like odd
- boj similar to boy
- ci again like tsee
  → roughly: OD‑boy‑tsee

The š is always a hard “sh” sound (like English sh in shoe).