Der Täter hat mein gestohlenes Fahrrad; deshalb erstatte ich Anzeige.

Because Fahrrad is neuter: das Fahrrad.

In this sentence, Fahrrad is the direct object of hat, so it is in the accusative case. For a neuter singular noun with a possessive word like mein, the form stays mein:

• nominative: mein Fahrrad
• accusative: mein Fahrrad

The adjective ending on gestohlen- changes too. After mein in neuter singular accusative, the adjective takes -es:

• mein gestohlenes Fahrrad

If the noun were masculine, you would get forms like:

• meinen gestohlenen Wagen

So the sentence is correct as written.

Gestohlenes is the past participle of stehlen used as an adjective.

• verb: stehlen = to steal
• past participle: gestohlen = stolen

When German uses a participle before a noun, it behaves like an adjective and gets an ending:

• das gestohlene Fahrrad = the stolen bicycle
• mein gestohlenes Fahrrad = my stolen bicycle

So gestohlenes is not acting as part of a verb tense here. It is simply describing Fahrrad.

No. Literally, it means The culprit has my stolen bicycle.

That is an important difference.

• Der Täter hat mein Fahrrad gestohlen. = The culprit stole my bicycle.
• Der Täter hat mein gestohlenes Fahrrad. = The culprit has my stolen bicycle.

In the first sentence, gestohlen is part of the verb phrase (hat gestohlen = has stolen / stole). In the second, gestohlenes is an adjective describing Fahrrad.

So word order changes the meaning a lot here.

Because Der Täter is the subject of the first clause.

German marks case on articles:

• der Täter = nominative, used for the subject
• den Täter = accusative, used for a masculine direct object

In Der Täter hat mein gestohlenes Fahrrad, the person doing the action of hat is der Täter, so nominative is needed.

Here, hat is just the present tense of haben = to have.

• Der Täter hat mein gestohlenes Fahrrad. = The culprit has my stolen bicycle.

It is not the perfect tense, because there is no second verb at the end.

Compare:

• Der Täter hat mein Fahrrad. = The culprit has my bicycle.
• Der Täter hat mein Fahrrad gestohlen. = The culprit has stolen / stole my bicycle.

So in your sentence, hat simply means has.

Because deshalb takes up the first position in the main clause, and in German main clauses the finite verb must come second.

So you get:

• deshalb erstatte ich Anzeige

not:

• deshalb ich erstatte Anzeige

German is a verb-second language in main clauses. The first position can be filled by many things, not only the subject. If deshalb comes first, the verb comes next, and the subject follows:

• Deshalb erstatte ich Anzeige.

You could also say:

• Ich erstatte deshalb Anzeige.

That is also grammatical, but the emphasis is slightly different.

Anzeige erstatten is a fixed, formal expression meaning:

• to report a crime
• to file a police report
• to press charges (depending on context)

It is common in police and legal contexts.

So Deshalb erstatte ich Anzeige means something like:

• That’s why I’m filing a police report.

It is more formal than everyday alternatives like:

• zur Polizei gehen = to go to the police
• einen Diebstahl melden = to report a theft

Because Anzeige erstatten is a fixed idiomatic expression, and in that expression Anzeige is usually used without an article.

So the standard phrasing is:

• Anzeige erstatten

not usually:

• eine Anzeige erstatten

Learners often want to translate word for word from English, but German often uses set verb-noun combinations that do not match English article usage exactly.

Also note that Anzeige can mean different things depending on context, for example an advertisement, but with erstatten it clearly means a formal report to the authorities.

Not exactly. Deshalb is an adverb, not a subordinating conjunction.

It means therefore, that’s why, or for that reason.

Because it is an adverb, it does not send the verb to the end. It behaves like a normal main-clause element and triggers verb-second order:

• Deshalb erstatte ich Anzeige.

Compare that with a subordinating conjunction like weil:

• ..., weil ich Anzeige erstatte.

So:

• deshalb = main clause word order
• weil = verb goes to the end in the subordinate clause

The semicolon links two closely related main clauses:

• Der Täter hat mein gestohlenes Fahrrad
• deshalb erstatte ich Anzeige

A semicolon shows a stronger break than a comma, but a weaker break than a full stop. It is a good choice here because the second clause is a direct consequence of the first.

You could also write:

• Der Täter hat mein gestohlenes Fahrrad. Deshalb erstatte ich Anzeige.

A comma is sometimes seen in similar sentences too, but the semicolon is clearer and a bit more formal.

Yes, it sounds fairly formal, especially because of:

• Täter
• Anzeige erstatten

Both are common in legal or police-related language.

More everyday German might use something like:

• Der Dieb hat mein Fahrrad, deshalb gehe ich zur Polizei.
• Mein Fahrrad wurde gestohlen, deshalb melde ich das der Polizei.

Your original sentence is perfectly understandable, but it has a legal/reporting tone.

Dieb means thief specifically.

Täter means perpetrator, offender, or culprit. It is broader and more formal. A Täter could be someone who committed many kinds of crimes, not just theft.

So:

• Der Dieb = the thief
• Der Täter = the perpetrator / offender

In police or legal language, Täter is very common, which is why it fits well with Anzeige erstatten.