Der Zahlungsbeleg liegt in der Originalverpackung.

Because Zahlungsbeleg is the subject of the sentence, so it is in the nominative case. The noun Beleg is masculine, so the nominative singular article is der.

A quick breakdown:

• der Beleg = the receipt / document / voucher
• der Zahlungsbeleg = the proof of payment / payment receipt

So:

• der Zahlungsbeleg = nominative masculine singular

Zahlungsbeleg is a compound noun, which is very common in German.

It is made of:

• Zahlung = payment
• Beleg = receipt, document, proof, record

So literally it is something like payment document or payment proof. In natural English, it often means:

• proof of payment
• payment receipt
• sometimes simply receipt, depending on context

Also, in German compounds, the last noun is the main one. That means:

• Beleg determines the gender
• because Beleg is masculine, Zahlungsbeleg is also masculine

German often uses liegen for something that is located somewhere, especially if it is understood as lying or resting in a place.

So:

• Der Zahlungsbeleg ist in der Originalverpackung = The receipt is in the original packaging.
• Der Zahlungsbeleg liegt in der Originalverpackung = The receipt is lying / is located in the original packaging.

In many situations, liegt sounds more natural than ist when talking about where a physical object can be found.

German often makes these distinctions:

• liegen = to lie, to be lying
• stehen = to stand, to be standing
• sitzen = to sit, to be sitting

English often just uses is.

Because the subject is singular:

• der Zahlungsbeleg = singular

The verb liegen is conjugated here in the 3rd person singular:

• ich liege
• du liegst
• er/sie/es liegt
• wir liegen
• ihr liegt
• sie/Sie liegen

So der Zahlungsbeleg liegt means the receipt lies / is located.

Because in is a two-way preposition in German. That means it can take:

• dative for location
• accusative for direction/movement toward

Here, the sentence describes where the receipt is located, not movement into the packaging. So German uses the dative:

• in der Originalverpackung = in the original packaging

Compare:

• Der Zahlungsbeleg liegt in der Originalverpackung.
  = It is in the packaging. (location → dative)

• Ich lege den Zahlungsbeleg in die Originalverpackung.
  = I put the receipt into the packaging. (movement/direction → accusative)

This is a very common question. In German, article forms change by case, so der does not always mean masculine nominative.

Here are some important forms of the:

• der = masculine nominative
• die = feminine nominative
• das = neuter nominative
• der = feminine dative
• der = feminine genitive
• der = plural genitive

So in in der Originalverpackung, the noun is:

• die Originalverpackung in the dictionary
• but after in with location, it becomes dative
• feminine dative singular = der Originalverpackung

So this der is not masculine here. It is feminine dative.

It is in the dative case.

Why?

• The preposition in is being used to show location
• Location with a two-way preposition takes the dative

So:

• dictionary form: die Originalverpackung
• in this sentence: in der Originalverpackung

That gives us:

• der = feminine dative singular article
• Originalverpackung = unchanged noun form here

Originalverpackung is another compound noun:

• original = original
• Verpackung = packaging

Together:

• Originalverpackung = original packaging / original box / original packing

German usually writes compound nouns as one word, not as separate words. So:

• Originalverpackung is correct
• writing it as separate words would usually be wrong in standard German

The main noun is Verpackung, which is feminine:

• die Verpackung
• therefore die Originalverpackung

Because all nouns are capitalized in German.

That is a normal spelling rule, not something special about this sentence.

So:

• der Zahlungsbeleg
• die Originalverpackung

Both words are nouns, so both begin with capital letters.

The sentence follows normal German main-clause word order:

• Der Zahlungsbeleg = subject
• liegt = verb
• in der Originalverpackung = prepositional phrase

So the pattern is:

• Subject + Verb + Other information

This is very typical in a simple German statement.

Also remember the main German rule:

• in a normal main clause, the finite verb must be in second position

That is exactly what happens here:

• position 1: Der Zahlungsbeleg
• position 2: liegt

Yes. That is correct.

• liegt sounds natural and concrete for a physical object
• befindet sich means something like is located and sounds a bit more formal or neutral

So both are possible:

• Der Zahlungsbeleg liegt in der Originalverpackung.
• Der Zahlungsbeleg befindet sich in der Originalverpackung.

In everyday use, liegt is often simpler and more idiomatic for an item that is physically there.

Not always. These words can overlap, but they are not identical.

• Zahlungsbeleg = proof of payment; a fairly broad term
• Quittung = receipt, often specifically a written acknowledgment that payment was received
• Kassenbon = till receipt / cash register receipt
• Rechnung = invoice / bill

So Zahlungsbeleg can be a more general or formal term. Depending on context, English might translate it as receipt, but German may choose a more specific word if needed.

Then you would use a verb of movement and the accusative after in.

For example:

• Ich lege den Zahlungsbeleg in die Originalverpackung.

Why?

• lege shows movement: I place it there
• in die Originalverpackung shows direction into the packaging
• so in takes the accusative

Compare the pair:

• Der Zahlungsbeleg liegt in der Originalverpackung.
  = The receipt is in the original packaging. (location)

• Ich lege den Zahlungsbeleg in die Originalverpackung.
  = I put the receipt into the original packaging. (movement)

Yes. It is in the present tense:

• liegt = present tense of liegen

In German, the present tense often covers what English expresses as:

• lies
• is lying
• is located

So this sentence can naturally describe the current state of where the receipt is.