Obwohl die Quittung fehlt, wurde das Buch zurückgebucht.

Because obwohl introduces a subordinate clause, and in German subordinate clauses the finite verb usually goes to the end.

So:

• Obwohl die Quittung fehlt = Although the receipt is missing

Here, fehlt is the conjugated verb, and it moves to the end because of obwohl.

Compare:

• Main clause: Die Quittung fehlt.
• Subordinate clause: ..., obwohl die Quittung fehlt.

In German, a subordinate clause is normally separated from the main clause with a comma.

So the structure is:

• Obwohl die Quittung fehlt,
• wurde das Buch zurückgebucht.

That comma is required because the sentence begins with a subordinate clause introduced by obwohl.

Obwohl means although or even though. It introduces a contrast:

• Obwohl die Quittung fehlt = Although the receipt is missing

It tells you that the second part happened despite the situation in the first part.

A useful point: obwohl is a subordinating conjunction, so it changes the word order by sending the finite verb to the end of its clause.

Because this sentence is in the passive voice.

German often forms the passive with:

• werden
  • past participle

Here:

• wurde = past tense of werden
• zurückgebucht = past participle

So:

• wurde das Buch zurückgebucht = the book was credited back / reversed / booked back

If it were active, you would need someone doing the action, for example:

• Die Bibliothek buchte das Buch zurück.

But the sentence does not focus on who did it. It focuses on what happened to the book.

That is normal German main-clause word order.

In a main clause, the finite verb goes in second position. Here, the first position is taken by the entire subordinate clause:

• Obwohl die Quittung fehlt, = position 1
• wurde = position 2

Then the rest of the clause follows, and the past participle goes to the end:

• wurde das Buch zurückgebucht

So the overall pattern is:

• subordinate clause first
• then finite verb in second position in the main clause
• participle at the end

Yes. In this passive sentence, das Buch is the subject.

In the active version, the book would usually be the object:

• Jemand buchte das Buch zurück.

In the passive, that object becomes the subject:

• Das Buch wurde zurückgebucht.

So in your sentence:

• das Buch is nominative singular
• it is the grammatical subject of the passive clause

That is why the article is das, not a different case form.

Because die Quittung is the subject of the subordinate clause:

• die Quittung fehlt

The verb fehlen works a bit differently from English to miss. German does not say the receipt is missed here. Instead, it says the receipt is missing / absent.

So:

• die Quittung = subject, nominative
• fehlt = is missing

That is why you do not see an accusative form here.

Yes. Zurückbuchen is a separable verb:

• base verb: buchen
• prefix: zurück-

The prefix zurück- often adds the idea of back or in reverse.

In different structures, separable verbs behave differently:

• Present tense main clause: Man bucht das Buch zurück.
• Infinitive: zurückbuchen
• Past participle: zurückgebucht

So zurückgebucht is the correct past participle form here.

Because German passive often leaves the agent unstated when it is unknown, unimportant, or obvious from context.

The sentence focuses on the result:

• the book was returned to the system / booked back

If you wanted to add the agent, you could say something like:

• ..., wurde das Buch von der Bibliothek zurückgebucht.

But in many real sentences, German simply omits that information.

Because the sentence is describing two time relationships:

• die Quittung fehlt = the receipt is missing
• wurde das Buch zurückgebucht = the book was booked back

This can mean that at the time being discussed, the receipt was missing, and despite that, the return was processed.

German often mixes tenses this way when one part describes a current or relevant condition and the other part describes a completed event.

Depending on context, this is completely natural.

Not in exactly the same structure.

• obwohl introduces a subordinate clause:

  • Obwohl die Quittung fehlt, wurde das Buch zurückgebucht.

• trotzdem is an adverb meaning nevertheless / even so:

  • Die Quittung fehlt. Trotzdem wurde das Buch zurückgebucht.

So both express contrast, but they work differently in the sentence.

A simple way to remember it:

• obwohl = conjunction, starts a subordinate clause
• trotzdem = adverb, belongs to a main clause

It could. German allows flexible word order, especially to highlight different information.

Your sentence starts with the obwohl clause to emphasize the surprising condition first:

• Obwohl die Quittung fehlt, wurde das Buch zurückgebucht.

You could also say:

• Das Buch wurde zurückgebucht, obwohl die Quittung fehlt.

Both are correct. The difference is mainly one of emphasis and information flow.

Not necessarily. Quittung is a normal word for receipt, especially in a general sense. Depending on context, German might also use words like:

• Beleg
• Kassenbon
• Bon

But Quittung is perfectly grammatical and natural here. The exact choice depends on context and register.