Bicikl je pronađen.

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

In Bicikl je pronađen, it works with pronađen to form a passive construction:

• je = is / has been / was
• pronađen = found

So the structure is basically subject + to be + passive participle.

Pronađen is the past passive participle of the verb pronaći (to find).

It agrees with the noun it describes:

• pronađen = masculine singular
• pronađena = feminine singular
• pronađeno = neuter singular
• pronađeni = masculine plural

Since bicikl is masculine singular, the correct form is pronađen.

In Croatian, every noun has grammatical gender. Bicikl is a masculine noun.

You can often guess masculine nouns because many of them end in a consonant, and bicikl does:

• bicikl → masculine

Because of that, any adjective or participle describing it must also be masculine singular:

• Bicikl je pronađen.

Yes. It is a passive sentence.

The active version would be something like:

• Netko je pronašao bicikl. = Someone found the bicycle.

The passive version:

• Bicikl je pronađen. = The bicycle was found / has been found.

Croatian uses this kind of passive fairly naturally, especially in formal or neutral statements.

It can mean either, depending on context.

Croatian does not match English tense usage exactly here. Bicikl je pronađen can be understood as:

• The bicycle was found
• The bicycle has been found

Context tells you which is more natural in English.

If this is an announcement after a missing bicycle was being searched for, English would often prefer has been found.
If it is part of a narrative about past events, was found may fit better.

Croatian has no articles like English a/an/the.

So bicikl can mean:

• a bicycle
• the bicycle

The context decides which one is meant.

In Bicikl je pronađen, English will often use the bicycle if the bicycle is already known from context.

Bicikl is in the nominative singular because it is the grammatical subject of the sentence.

In a passive sentence, the thing affected by the action becomes the subject:

• active: Netko je pronašao bicikl.
  Here bicikl is the object.
• passive: Bicikl je pronađen.
  Here bicikl becomes the subject, so it is nominative.

Yes. Croatian word order is more flexible than English.

You can also say:

• Pronađen je bicikl.

Both are correct, but the emphasis changes slightly.

• Bicikl je pronađen. → more neutral, starting with the bicycle
• Pronađen je bicikl. → puts more focus on the fact that it was found

The version with Bicikl first is a very straightforward default sentence.

Croatian often places short unstressed words, called clitics, in the second position of the clause. Je is one of them.

So in:

• Bicikl je pronađen.

the clitic je comes after the first element, Bicikl.

This is a very common pattern in Croatian sentence structure.

The base verb is pronaći, which is a perfective verb meaning to find.

Perfective verbs usually present an action as completed. That fits this sentence well, because the bicycle has already been found.

A related imperfective verb is:

• pronalaziti = to be finding / to find repeatedly / habitually

For this completed-result sentence, pronađen from pronaći is the natural choice.

Approximately: pro-NAH-jen.

A few details:

• pro as in pro-duce, but shorter
• stress is usually on na
• đ is a soft sound somewhat like the j in duke or during in some English accents

So pronađen is roughly pro-NAH-jen.

Yes. Nađen is also used and comes from naći (to find), which is closely related to pronaći.

So both are possible:

• Bicikl je nađen.
• Bicikl je pronađen.

Very roughly:

• naći / nađen = find / found
• pronaći / pronađen = often feels a bit more like locate / discover / find

In many contexts, the difference is small, and both are natural.