Learn Hardcore Croatian: Onaj čovjek u parku je moj ujak.

Onaj is a demonstrative word, and here it means that (man), usually with a sense of some distance (physically or in context), similar to that man (over there).

Croatian has three basic demonstratives that roughly match English this / that distinctions:

ovaj – this (near the speaker)
taj – that (near the listener or already mentioned)
onaj – that (over there), more distant or clearly singled out

So onaj čovjek is not just the man, but more specifically that man.
If you simply wanted to say The man in the park is my uncle, you could say:

Čovjek u parku je moj ujak. (no demonstrative)

Why is parku used instead of park in u parku?

Parku is the locative singular form of park.

In Croatian, many prepositions require a specific grammatical case. The preposition u:

with the meaning in / inside normally takes the locative case.

So:

dictionary form (nominative): park
locative singular: parku

Hence u parku literally means in (the) park and is grammatically preposition + locative.

What case are čovjek and ujak in, and why are they the same form?

Both čovjek (man) and ujak (maternal uncle) are in the nominative singular.

With the verb biti (to be – here je), when you equate two things (X is Y), both sides are in the nominative:

Onaj čovjek u parku – nominative (subject)
je – third person singular of biti
moj ujak – nominative (predicative complement)

This is like saying in English: That man in the park = my uncle.
Both noun phrases are grammatically “subject-like” around the verb to be, so they stay in the base (nominative) form.

What exactly is je, and why is it in the middle of the sentence?

Je is the 3rd person singular present form of biti (to be). It means is.

In sentences like this, je is a clitic (an unstressed short word) and tends to appear in the second position of the clause or after the first “chunk” (which can be a phrase rather than a single word). In this sentence:

First phrase: Onaj čovjek u parku
Second-position clitic: je
Rest of sentence: moj ujak

So you get: Onaj čovjek u parku je moj ujak.

You will often see je right after the first word or phrase, not always exactly where is would go in English.

Can I move parts of the sentence around, for example say Moj ujak je onaj čovjek u parku?

Yes. Croatian word order is more flexible than English because grammatical roles are marked by case endings rather than word position.

These two are both correct:

Onaj čovjek u parku je moj ujak. – Neutral: you’re identifying who that man is.
Moj ujak je onaj čovjek u parku. – Slightly different focus: you’re identifying which man your uncle is.

The meaning is essentially the same, but the focus can shift:

First version: focus on that man (“That man in the park — he’s my uncle.”)
Second version: focus on my uncle (“My uncle — he’s that man in the park.”)

Can I drop je, like Onaj čovjek u parku moj ujak?

In standard Croatian, no — you should not drop je in this kind of sentence. The copula (to be) is normally required:

✔ Onaj čovjek u parku je moj ujak.
✘ Onaj čovjek u parku moj ujak. (ungrammatical in standard language)

In some dialects or very colloquial speech, people might occasionally omit je in certain contexts, but as a learner you should always include it.

What’s the difference between ujak, stric, and tetak? They all seem to mean uncle.

Croatian is more precise than English about relatives on the mother’s vs father’s side and by marriage:

ujak – your mother’s brother (maternal uncle)
stric – your father’s brother (paternal uncle)
tetak – the husband of your aunt (aunt’s husband)

In this sentence, ujak specifically tells us that the man is the speaker’s mother’s brother. English just says uncle, but Croatian specifies the relationship.

Why is there no word for the in this sentence?

Croatian has no articles like English a / an / the.

Definiteness (whether something is specific/known) is shown mainly by:

context
demonstratives (ovaj, taj, onaj, etc.)
word order and emphasis

In Onaj čovjek u parku je moj ujak, definiteness is handled by onaj (that man).

If you removed onaj and said:

Čovjek u parku je moj ujak.

it would still usually be understood as The man in the park is my uncle from context, even though there is no the.

Why is the phrase u parku placed after čovjek and not before, like u parku čovjek?

In Croatian, modifiers like prepositional phrases (e.g. u parku) usually follow the noun they describe:

čovjek u parku – literally man in (the) park

Putting u parku before čovjek (u parku čovjek) would sound incomplete or marked, and would usually need extra structure, like:

U parku je čovjek. – There is a man in the park.

So within a noun phrase, you normally say:

čovjek u parku, dječak u školi, auto na cesti, etc.

How do you pronounce čovjek and ujak?

Approximate pronunciations (in English-friendly terms):

čovjek ≈ CHO-vyek
- č = like ch in church
- o = like o in not (short)
- vjek: v
  - yeck (but shorter and smoother; j = English y)
ujak ≈ OO-yak
- u = like oo in boot (shorter)
- j = English y in yes
- stress usually on the first syllable: Ú-jak, ČÓ-vjek (exact stress patterns can vary slightly by dialect, but this is a good approximation).

How would the sentence change if the person were a woman instead of a man?

You’d need to change:

the demonstrative (onaj → ona)
the noun (čovjek → a feminine noun, e.g. žena, gospođa)
the possessive (moj → moja) if the relative word is feminine (e.g. tetka)

Examples:

Ona žena u parku je moja tetka.
- That woman in the park is my aunt.
Ona gospođa u parku je moja teta.
- That lady in the park is my aunt.

Notice the gender agreement:

masculine: onaj čovjek / moj ujak
feminine: ona žena / moja tetka

Could I say Onaj ujak u parku je moj ujak or is that strange?

It’s grammatically fine but usually odd in meaning:

Onaj ujak u parku je moj ujak. literally: That uncle in the park is my uncle.

You normally use čovjek (man) or some neutral description for someone you’re pointing out who is not yet identified as your relative:

Onaj čovjek u parku je moj ujak. – That man in the park is my uncle.

Saying onaj ujak already labels him as an uncle from the listener’s point of view, so repeating moj ujak becomes redundant or emphasizes something unusual (like distinguishing between multiple uncles).