Koja je marka bicikla?

Because koja comes from koji, which means which or what kind of when it agrees with a noun.

Here, the noun being asked about is marka (brand), so Croatian uses koja to match it:

• koji = masculine
• koja = feminine
• koje = neuter

Since marka is feminine, the question uses koja.

So the structure is basically:

• Koja je marka ...? = What/which brand is ...?

Using što here would not sound natural, because što is usually what as a standalone pronoun, not an adjective agreeing with marka.

Because it agrees with marka, and marka is a feminine noun in Croatian.

Croatian adjectives and adjective-like question words change form to match the noun they describe in:

• gender
• number
• case

So:

• koji bicikl = which bicycle
• koja marka = which brand
• koje vino = which wine

In this sentence, koja matches marka, not bicikla.

Because bicikla is the genitive singular form of bicikl.

• nominative: bicikl = bicycle
• genitive: bicikla = of the bicycle / of a bicycle

In marka bicikla, the genitive shows a relationship like the brand of the bicycle.

This is very common in Croatian:

• boja auta = the color of the car
• vrata kuće = the door of the house
• marka bicikla = the brand of the bicycle

So bicikla is not a different word; it is just the correct case form.

Croatian has no articles, so bicikla by itself does not tell you whether English should use the or a.

Depending on context, marka bicikla can mean:

• the brand of the bicycle
• the brand of a bicycle

In real use, this sentence often refers to a specific bike that both speakers can identify, so English usually translates it as What brand is the bicycle? or more naturally What brand is the bike?

Because je is a clitic in Croatian. Clitics are short unstressed words that usually go in second position in the sentence.

So in:

• Koja je marka bicikla?

the first element is Koja, and je comes right after it.

This second-position rule is very common in Croatian:

• Gdje je bicikl? = Where is the bicycle?
• Tko je to? = Who is that?
• Koja je marka? = What brand is it?

For learners, it is useful to remember that forms like je, sam, si, se often want to sit near the beginning of the sentence.

Yes. A very common alternative is:

• Koje je marke bicikl?

This also means What brand is the bicycle/bike?

The two versions are closely related:

• Koja je marka bicikla? = literally What is the brand of the bicycle?
• Koje je marke bicikl? = literally Of what brand is the bicycle?

Both are natural. The second one is especially common in everyday speech.

Here marka means brand, make, or manufacturer label.

So if someone asks:

• Koja je marka bicikla?

they want to know whether it is, for example:

• Trek
• Giant
• Specialized
• Cube

In some contexts, English make might sound more natural than brand, but marka is the normal Croatian word here.

Some natural answers are:

• Marka bicikla je Trek. = The brand of the bicycle is Trek.
• Bicikl je marke Trek. = The bicycle is of the brand Trek.
• To je Trek. = It’s a Trek.

In everyday speech, shorter answers are very common:

• Trek.
• Giant.
• Cube.

A simple pronunciation guide is:

• Koja je marka bicikla?
• KO-ya ye MAR-ka bee-TSI-kla?

A few helpful notes:

• j in Croatian sounds like English y
• je sounds roughly like ye
• c sounds like ts
• koja sounds like KO-ya

So bicikla is approximately bee-TSI-kla, not bi-SI-kla.

It is neutral and completely normal in both formal and informal situations.

You could use it:

• in a shop
• in conversation with a friend
• when describing an object
• in writing

If you want a more conversational version, many speakers might say:

• Koje je marke bicikl?

But Koja je marka bicikla? is absolutely correct and natural.