Der Reservierungsbeleg hilft insofern, als er die Reservierung nachweist.

Insofern, als is a somewhat formal connector meaning insofar as, to the extent that, or in that.

So the sentence means something like:

• The reservation receipt is helpful insofar as it proves the reservation.
• more naturally in English: The reservation confirmation/receipt helps because it serves as proof of the reservation.

Important point:

• insofern introduces the idea of in this respect / to this extent
• als then introduces the clause that explains in what way

This is more formal than simply saying weil (because).

Because als er die Reservierung nachweist is a subordinate clause, and German normally separates subordinate clauses with a comma.

So the structure is:

• main clause: Der Reservierungsbeleg hilft insofern
• subordinate clause: als er die Reservierung nachweist

German punctuation is stricter than English here: the comma is required.

Because the clause after als is a subordinate clause, and in German subordinate clauses the conjugated verb usually goes to the end.

Compare:

• main clause: Er weist die Reservierung nach.
• subordinate clause: ..., als er die Reservierung nachweist.

This is a very common German pattern:

• weil er kommt
• dass sie arbeitet
• obwohl es regnet
• als er die Reservierung nachweist

So the final position of nachweist is caused by the subordinate clause word order.

It is a compound noun:

• Reservierung = reservation
• Beleg = receipt, document, proof, voucher

Together, Reservierungsbeleg means something like:

• reservation receipt
• reservation document
• proof of reservation

German often forms long nouns by combining smaller nouns into one word.

Also, the final part usually determines the grammatical gender, so because der Beleg is masculine, we get:

• der Reservierungsbeleg

In German compound nouns, the last element determines the gender.

Here the last element is Beleg, and that noun is masculine:

• der Beleg

So the full compound is also masculine:

• der Reservierungsbeleg

This is a very useful rule for compound nouns in German.

Because er is masculine singular, and it agrees with der Reservierungsbeleg, which is masculine.

• der Reservierungsbeleg → masculine → er
• die Reservierung → feminine → would be sie

So in this sentence:

• als er die Reservierung nachweist

er can only refer to der Reservierungsbeleg, not die Reservierung.

This is one reason German pronouns can sometimes be clearer than English pronouns.

Here nachweisen means to prove, to demonstrate, or to provide evidence for.

So:

• die Reservierung nachweisen = to prove the reservation / to show evidence of the reservation

It is a very common word in formal or practical contexts, especially when talking about documents, evidence, or proof.

Examples:

• Er kann seine Identität nachweisen. = He can prove his identity.
• Sie müssen die Zahlung nachweisen. = You must provide proof of payment.

Yes. Nachweisen is a separable verb.

That means:

• infinitive: nachweisen
• main clause: Er weist die Reservierung nach.
• subordinate clause: ..., als er die Reservierung nachweist.

So in a main clause, the prefix nach- splits off:

• weist ... nach

But in a subordinate clause or infinitive, it stays together:

• nachweist
• nachweisen

This is a very common pattern with separable verbs.

Die Reservierung is in the accusative case because it is the direct object of nachweisen.

The verb asks: What does he/it prove? Answer: die Reservierung

Since Reservierung is feminine, nominative and accusative both look the same:

• nominative: die Reservierung
• accusative: die Reservierung

So the form does not change, but the function is accusative.

Because the subject is singular:

• Der Reservierungsbeleg = the reservation receipt/document

That is one thing, so the verb is singular:

• Der Reservierungsbeleg hilft.

If the subject were plural, the verb would also be plural:

• Die Reservierungsbelege helfen.

Because insofern is not a noun here. It is an adverb or connector meaning in that respect / to that extent.

So it does not take an article. It functions more like a linking word than like an object.

Think of it as part of the expression:

• insofern, als ...

not as something like:

• in dem Sofern or anything similar

It is a fixed expression.

It is the same word in form, but here it has a different function.

Als can have several uses in German, for example:

• größer als = bigger than
• als ich jung war = when I was young
• insofern, als ... = insofar as / in that

So in your sentence, als is part of the formal connector insofern, als. It does not mean than, and it does not mean when.

You have to understand it from the whole phrase, not as an isolated word.

Yes, in many situations you could rephrase it with weil, but the meaning would shift slightly.

For example:

• Der Reservierungsbeleg hilft, weil er die Reservierung nachweist.

This means:

• The reservation receipt helps because it proves the reservation.

That is perfectly understandable and more straightforward.

But insofern, als is a bit more precise and formal. It suggests:

• it helps specifically in the sense that
• it helps to the extent that

So weil gives a simple reason, while insofern, als highlights the specific respect in which something is true.

It sounds somewhat formal, administrative, or written.

Especially formal are:

• Reservierungsbeleg
• insofern, als
• nachweisen

In everyday speech, many speakers might say something simpler, such as:

• Der Reservierungsbeleg hilft, weil er zeigt, dass die Reservierung existiert.
• Der Beleg ist hilfreich, weil er die Reservierung bestätigt.

So the original sentence is natural, but it has a more official or written tone.

Sometimes yes, depending on context, but not always exactly.

Beleg often suggests:

• receipt
• voucher
• documentary proof

So Reservierungsbeleg is something like a document showing that a reservation exists.

In real-life English, a natural translation might be:

• reservation confirmation
• reservation receipt
• proof of reservation

Which one is best depends on the situation:

• hotel booking
• restaurant reservation
• travel reservation
• payment-related document

So the German word is a bit broader than one single English equivalent.

Yes, that is a very useful way to read it.

Part 1:

• Der Reservierungsbeleg hilft insofern
• The reservation receipt helps in this respect

Part 2:

• als er die Reservierung nachweist
• namely, by proving the reservation

So the overall logic is:

1. the document is helpful
2. the sentence explains exactly how or in what respect it is helpful

That is the role of insofern, als in the sentence.