Koji je ovo model bicikla?

Here koji means which, but in English this often gets translated as what in phrases like What model is this?

Croatian uses koji before a noun when you are asking someone to identify one item from a set: koji model, koja knjiga, koje auto.
Što means what, but it is a standalone pronoun, not the normal word to put directly before a noun in this kind of question.

So:

• Koji je ovo model bicikla? = What/which model of bicycle is this?
• not Što je ovo model bicikla?

Because it agrees with model, which is a masculine singular noun.

In Croatian, words like koji change form to match the noun they refer to in:

• gender
• number
• case

Here:

• model = masculine singular
• so the question word is koji

Compare:

• Koji model? = masculine singular
• Koja marka? = feminine singular
• Koje vozilo? = neuter singular

Because je is a clitic in Croatian. Clitics usually go in the second position in the sentence or clause.

So in:

• Koji je ovo model bicikla?

the word je comes very early, after the first stressed element, which is koji.

This is very normal Croatian word order. English speakers often expect something more like Koji ovo je..., but that is not how standard Croatian works.

Because ovo here is not directly describing model as an adjective. It is functioning more like a pronoun meaning this one / this thing here.

So the structure is roughly:

• Which model is this?

That is why Croatian uses ovo, the neuter form commonly used for this when pointing to something.

If you used ovaj, that would normally directly modify a masculine noun:

• ovaj model = this model

But in Koji je ovo model bicikla?, the speaker is pointing to something and asking for its model, so ovo is the natural form.

Because bicikla is the genitive singular form of bicikl.

Croatian often uses a noun + genitive structure where English uses either:

• of
• or a noun used like an adjective

So:

• model bicikla literally = model of a bicycle
• natural English = bicycle model or bike model

This is a very common pattern in Croatian:

• vrata auta = the car door
• boja zida = the color of the wall
• marka automobila = the make/brand of the car

Model is in the nominative singular.

That is because it is the predicate noun after je in a sentence with biti (to be). In standard Croatian, predicate nouns in this kind of sentence are normally in the nominative.

So the basic structure is:

• Koji = interrogative word agreeing with model
• je = is
• ovo = this
• model bicikla = bike model

That is an important difference.

• Koji asks for identity: which one exactly?
• Kakav asks for type, nature, or characteristics: what kind?

So:

• Koji je ovo model bicikla? = You want the exact model name or designation.
• Kakav je ovo bicikl? = You want a description, such as mountain bike, road bike, cheap, fast, old-fashioned, etc.

In many cases where English says what model, Croatian prefers koji model.

Yes, Croatian word order is fairly flexible, but not completely free. The main restriction here is that je still behaves like a clitic.

A possible alternative is:

• Koji model bicikla je ovo?

This is also natural and may even feel more straightforward to an English speaker, because it starts with the whole noun phrase koji model bicikla.

Both versions are fine:

• Koji je ovo model bicikla?
• Koji model bicikla je ovo?

The meaning is essentially the same.

Yes, it is natural.

A speaker might use it when pointing at a bicycle and asking someone to identify the model. Depending on context, people might also shorten it a bit, for example:

• Koji je ovo model? if it is already obvious you are talking about a bicycle
• Koji model je ovo? in casual speech

But Koji je ovo model bicikla? is a perfectly normal full sentence.