Learn Hardcore Croatian: Njegov brat često trči u parku.

Croatian has no articles (a/the), so if you say just Brat često trči u parku, it sounds like a very general statement: The brother often runs in the park (with no clear idea whose brother it is).
If you want to say specifically his brother, you must use njegov.

What exactly does njegov refer to, and does it change form?

Njegov is a possessive pronoun meaning his. It refers to some male person previously mentioned in the context (e.g. Marko → Njegov brat = Marko’s brother).

It agrees with the noun it describes in gender, number, and case:

njegov brat – his brother (masc. sg.)
njegova sestra – his sister (fem. sg.)
njegovo dijete – his child (neut. sg.)
njegovi prijatelji – his friends (masc. pl.)

So the form changes depending on what is possessed, not on who the “he” is.

Where is the word for he in this sentence?

There is no separate word for he here, because Croatian often drops subject pronouns when the subject is clear.

In this sentence, the subject is explicitly njegov brat (his brother), so you don’t need on (he).
Saying On njegov brat često trči u parku would sound wrong, like saying He his brother often runs in the park in English.

What verb is trči from, and how is it conjugated?

Trči is the 3rd person singular present of the verb trčati = to run.

Present tense of trčati:

ja trčim – I run
ti trčiš – you run (sg.)
on/ona/ono trči – he/she/it runs
mi trčimo – we run
vi trčite – you run (pl./formal)
oni/one/ona trče – they run

So njegov brat trči = his brother runs.

Is trčati perfective or imperfective? How would I say “ran (once) in the park”?

Trčati is imperfective – it describes an ongoing or repeated action (running in general, often, regularly).

For a single, completed action, you usually need a perfective verb, often made with a prefix, e.g.:

otrčati, istrčati, potrčati – all are perfective, but each has its own nuance (run off, run out, start running, etc.).

For He ran in the park you would most commonly say:

On je trčao u parku. (imperfective, describes what he was doing / used to do)

If you want to stress a one-time, completed event, you’d pick the perfective verb that fits the situation (e.g. Otrčao je u park. = He ran (off) to the park).

Why is it često trči and not trči često? Can često go in other positions?

Često means often.

The most neutral, common order is:

Njegov brat često trči u parku.

You can say:

Njegov brat trči često u parku. – still possible, but it sounds less natural and may put a bit more emphasis on često.

Other options:

Često njegov brat trči u parku. – marked word order, used for emphasis on “often”, and sounds more poetic or stylistically special.

In everyday speech, keep često before the verb: često trči.

Why do we say u parku and not u park?

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

u + locative → location: in, inside
- u parku = in the park (where he runs)
u + accusative → direction: into, to
- u park = into the park (where he is going)

Your sentence describes where he runs, not where he’s going, so you use the locative: u parku.

What case is parku, and how does the noun park decline?

Parku is locative singular of park (a regular masculine noun).

Singular of park:

Nominative: park – the park
Genitive: parka – of the park
Dative: parku – to/for the park
Accusative: park – (into) the park
Vocative: parke – O park (rarely used)
Locative: parku – in/on/at the park
Instrumental: parkom – with/by the park

So u parku = in the park.

Why is there no word for the before brat or park?

Croatian has no articles like a/an or the.

brat can mean a brother or the brother depending on context.
park can mean a park or the park.

The specificity (a vs the) is usually understood from the situation or from additional words, e.g. taj park – that park, ovaj park – this park.

What’s the difference between u parku and po parku? Could I say Njegov brat često trči po parku?

Yes, you can say Njegov brat često trči po parku, but there is a nuance:

u parku – in the park (just location, neutral)
po parku – around/throughout the park (suggests movement over the area, here and there)

So po parku can hint more at running around all over/around the park, not just being in it. Both are correct; u parku is more neutral.

How do you pronounce the č in često and trči?

The letter č is a single sound, similar to ch in church but usually a bit “harder”.

In your sentence:

često – pronounced roughly “chesto”
trči – pronounced roughly “trr-chi” (rolled r, then č = ch in church)

Every letter in Croatian is pronounced, and each letter has one main sound, so č is always that ch sound.