Die Taxiquittung dient als Kostennachweis.

German very often combines nouns into a single compound noun.

Here:

• Taxi = taxi
• Quittung = receipt

So Taxiquittung literally means taxi receipt.

A useful rule: in German compounds, the last part is the main noun. That means:

• Taxiquittung is a kind of Quittung
• its gender is the same as Quittung
• its plural is built from Quittung as well

This is extremely common in German:

• Kostennachweis = proof/documentation of costs
• Hotelrechnung = hotel bill
• Fahrkarte = travel ticket

Because Quittung is a feminine noun: die Quittung.

Since Taxiquittung is a compound whose final element is Quittung, the whole word is also feminine:

• die Taxiquittung

In this sentence, it is also the subject, so it is in the nominative case.

Dient is the 3rd person singular present tense of dienen.

The subject is die Taxiquittung, which is singular, so the verb must be singular too:

• die Taxiquittung dient

Compare:

• Die Quittung dient ... = The receipt serves ...
• Die Quittungen dienen ... = The receipts serve ...

So the ending -t is just the normal present-tense ending for er/sie/es.

Here dienen means to serve as or to function as.

A very common pattern is:

• etwas dient als + noun

So:

• Die Taxiquittung dient als Kostennachweis.
• The taxi receipt serves as proof of costs.

This is a bit formal and is especially common in office, business, insurance, and reimbursement language.

You might also see:

• Der Ausweis dient als Identitätsnachweis.
• Diese E-Mail dient als Bestätigung.

Because als is the normal word used to show a role or function after verbs like dienen.

So:

• als Kostennachweis = as proof of costs

This is the idiomatic pattern.
A learner might wonder about wie or für, but they do not work the same way here:

• als = as, in the role/function of
• wie = like, as in comparison
• für = for

So dient als is the correct combination.

After als, German often uses a noun without an article when talking about someone's or something's function, role, or classification in a general sense.

So:

• als Kostennachweis
• als Beweis
• als Bestätigung
• als Lehrer

This is similar to saying as proof, as confirmation, as a teacher, where the focus is on the role rather than on one specific item.

If you added an article, it would usually sound more specific or less natural in this context.

It is another compound noun:

• Kosten = costs / expenses
• Nachweis = proof, evidence, documentation

So Kostennachweis means something like:

• proof of costs
• evidence of expenses
• expense documentation

This is the kind of word you often meet in formal or bureaucratic German.

In this sentence, Kostennachweis is part of a predicative expression with als. It refers back to the subject die Taxiquittung and describes its function.

Because there is no article, you cannot actually see a case ending here. But functionally, it is tied to the subject, not to a direct object.

For learners, the most important thing is not to think of als here as introducing a normal object. Instead, think of the whole phrase:

• dient als + noun

as a set pattern meaning serves as + noun.

Yes, that is possible, but the meaning shifts slightly.

• Die Taxiquittung ist ein Kostennachweis.
  = The taxi receipt is a proof/document of expenses.

• Die Taxiquittung dient als Kostennachweis.
  = The taxi receipt serves as proof/document of expenses.

The version with dient als emphasizes function or use.
That makes it especially natural in administrative or practical contexts, such as reimbursement, accounting, or travel expenses.

Yes, it sounds fairly formal and administrative.

You are likely to see this kind of sentence in contexts such as:

• expense claims
• travel reimbursement
• tax documents
• company policies

In everyday speech, someone might say something simpler depending on context, but this sentence is perfectly natural in formal German.