Learn Hardcore Croatian: On trči brzo kao moj brat.

Full form (rarely said): On trči brzo kao što moj brat trči. – He runs fast like my brother runs.
Normal, natural form: On trči brzo kao moj brat. – He runs fast like my brother (does).

The verb trči is understood after moj brat, so it doesn’t need to be repeated. This is very common and correct in Croatian.

Is kao more like English like or as?

kao can correspond to both like and as, depending on the sentence:

On trči brzo kao moj brat.
– He runs fast *like my brother. / He runs **as fast as my brother (does).*
Radi kao učitelj.
– He works *as a teacher.* (here it means “in the role of”)

In comparisons of manner/degree (like my brother, as fast as my brother), kao is the standard word. Sometimes it’s closer to like, sometimes to as, but Croatian doesn’t clearly separate them the way English does.

Is the word order fixed? Could I say On brzo trči kao moj brat instead?

Word order in Croatian is flexible, but there are “more natural” preferences.

On trči brzo kao moj brat. – neutral, very natural.
On brzo trči kao moj brat. – also possible; puts a bit more emphasis on brzo (the quickness of the action).

Both are grammatically correct. In casual speech you’ll most often hear On trči brzo…, but swapping trči and brzo is allowed and not wrong.

Why is it moj brat and not moga brata or mog brata? Which case is this?

moj brat is in the nominative case (the basic “dictionary form” of the noun).

After kao in a comparison like this, Croatian usually uses the nominative, because it’s understood as a shortened clause:

On trči brzo kao moj brat (trči).
– literally: He runs fast as my brother runs.

The implied subject of the missing verb (brat) stays in nominative: moj brat.

moga/mog brata would be genitive, which you’d see after od (“than/from” in comparisons):

On trči brže od mog brata. – He runs faster than my brother.

So:

kao moj brat → nominative
od mog brata → genitive

How do I say clearly “He runs as fast as my brother,” not just “fast like my brother”?

To make the equality of speed explicit, Croatian often adds a word like jednako or isto tako:

On trči jednako brzo kao moj brat.
– He runs as fast as my brother.
On trči isto tako brzo kao moj brat.
– He runs just as fast as my brother.

The original sentence On trči brzo kao moj brat. will often be understood roughly the same way in context, but adding jednako or isto tako makes the equality in degree very clear.

Can I drop On and just say Trči brzo kao moj brat.?

Yes. Croatian is a pro-drop language, which means subject pronouns are often omitted when the verb ending already shows the person:

On trči brzo kao moj brat. – He runs fast like my brother.
Trči brzo kao moj brat. – same meaning, He runs fast like my brother, but he is understood from context.

You usually keep on for:

contrast: On trči brzo, a ona trči polako. – He runs fast, but she runs slowly.
emphasis: Baš on trči brzo. – He, specifically, runs fast.

In a neutral sentence, leaving it out is very natural.

What’s the difference between brz and brzo? Why is it brzo here?

brz – adjective: fast (describes a noun)
- brz auto – a fast car
- brz pas – a fast dog
brzo – adverb: fast / quickly (describes a verb)
- On trči brzo. – He runs fast / quickly.

In On trči brzo kao moj brat., brzo modifies the verb trči (runs), so the adverb form is correct.

If you were describing a noun, you’d use brz:

On je brz kao moj brat. – He is fast like my brother.

What tense is trči? Does it mean “runs” or “is running”?

trči is the present tense (3rd person singular) of trčati.

In Croatian, the present tense usually covers both English:

He runs (habitual)
He is running (right now)

So On trči brzo kao moj brat. could mean either:

He (generally) runs fast like my brother.
He (right now) is running fast like my brother.

Context will decide which one is meant. You don’t normally need a separate continuous form in Croatian.

How would I say “He runs faster than my brother” using this sentence as a starting point?

You need the comparative form of brzo and the preposition od:

On trči brže od mog brata.

Breakdown:

brže – faster (comparative of brzo)
od – than / from
mog brata – genitive of moj brat (of my brother)

So: On trči brže od mog brata. – He runs faster than my brother.

Why is it moj brat and not something like moja brat or moje brat? How does moj agree with brat?

In Croatian, possessive pronouns agree in gender, number, and case with the noun they modify.

brat – masculine, singular, nominative

So you need the masculine singular nominative form of my, which is moj:

moj brat – my brother

Compare with other genders:

moja sestra – my sister (feminine singular)
moje dijete – my child (neuter singular)
moji prijatelji – my friends (masculine plural)

In On trči brzo kao moj brat., moj brat is nominative (as explained before), so moj is the correct form.