Bicikl je ukraden.

Croatian does not have articles, so there is no direct equivalent of the or a/an in normal sentences.

So bicikl can mean:

• a bicycle
• the bicycle

Which one is meant depends on context.

In Bicikl je ukraden, the context usually makes it clear whether you mean the bicycle or just a bicycle.

Bicikl is in the nominative singular.

That is because it is the subject of the sentence:

• Bicikl = the thing being talked about
• je ukraden = is/was stolen

The basic dictionary form is also bicikl, so in this sentence the nominative looks the same as the citation form.

Je is the 3rd person singular present tense of the verb biti (to be).

So:

• ja sam = I am
• ti si = you are
• on/ona/ono je = he/she/it is

In this sentence, je is used together with ukraden to form a passive/resultative expression:

• Bicikl je ukraden.

Ukraden is the past passive participle of the verb ukrasti (to steal).

It means something like:

• stolen

So the structure is roughly:

• bicikl = bicycle
• je = is
• ukraden = stolen

Together: The bicycle is/has been/was stolen, depending on context.

Because ukraden agrees with bicikl, which is:

• masculine
• singular
• nominative

In Croatian, this participle behaves a lot like an adjective, so it changes form to match the noun.

Compare:

• Bicikl je ukraden. — masculine singular
• Knjiga je ukradena. — feminine singular
• Pismo je ukradeno. — neuter singular
• Bicikli su ukradeni. — masculine plural

This is a very common thing learners notice.

In Croatian, je + past passive participle often expresses a completed action with a present result, and in many contexts English translates that naturally as was stolen or has been stolen.

So:

• Bicikl je ukraden.

can often correspond to English:

• The bicycle was stolen.
• The bicycle has been stolen.

The exact English tense depends on context. Croatian does not always match English tense choice one-for-one here.

Yes, it is a passive-type construction.

The focus is on the bicycle, not on who stole it.

If you want the active version, you could say:

• Netko je ukrao bicikl. — Someone stole the bicycle.

So:

• Bicikl je ukraden. = passive / result-focused
• Netko je ukrao bicikl. = active / doer-focused

Yes. Croatian word order is fairly flexible.

The neutral order is:

• Bicikl je ukraden.

But you may also hear:

• Ukraden je bicikl.

That version puts more focus or emphasis on ukraden or on the fact of the theft itself.

Both are grammatical, but Bicikl je ukraden is the most straightforward default order.

Normally, no. In a full standard sentence, you need je:

• Bicikl je ukraden.

Without je, it sounds incomplete in ordinary speech and writing:

• Bicikl ukraden. — not a normal full sentence

However, in headlines, notes, police-style reports, or very compressed informal language, Croatian sometimes drops auxiliaries. But for normal learning purposes, keep je.

The auxiliary and participle must agree with the subject.

Examples:

• Knjiga je ukradena. — The book was stolen.
• Auto je ukradeno. — The car was stolen.
• Bicikli su ukradeni. — The bicycles were stolen.
• Torbe su ukradene. — The bags were stolen.

Notice:

• singular: je
• plural: su

And the participle changes too:

• ukraden
• ukradena
• ukradeno
• ukradeni / ukradene

It can, but it usually adds a different nuance.

• Bicikl je ukraden. is the normal, simple way to say that the bicycle has been / was stolen.
• Bicikl je bio ukraden. sounds more like the bicycle had been stolen or emphasizes a past state/result in a larger past context.

So in most ordinary situations, the simpler and more natural sentence is:

• Bicikl je ukraden.