Der Hosenbund endet an der Seitennaht.

Hosenbund is a compound noun: Hosen + Bund.

In German, the last part of a compound noun decides the gender.
Here, the last part is Bund, which is masculine: der Bund.

So the whole word is:

• der Hosenbund = the waistband

That is why the sentence starts with Der Hosenbund.

This is a common feature of German compounds. The first part of a compound does not always look exactly like the basic dictionary form.

Here, Hosen- comes from die Hose. In compounds, German often uses a form like Hosen- for clothing-related words, for example:

• Hosenbein = trouser leg / pant leg
• Hosentasche = pants pocket
• Hosenbund = waistband

So Hosen- here is just the normal compound form. You do not need to treat it as a separate plural noun in the sentence.

German and English do not always divide things up the same way.

In standard German, die Hose is usually a singular noun meaning a pair of trousers / a pair of pants. So:

• die Hose = a pair of trousers
• die Hosen = trousers / pants in the plural, or several pairs

That is why the compound is based on Hose, even though English speakers often think of pants as plural.

The subject is der Hosenbund, which is third person singular.

So the verb enden has to be conjugated accordingly:

• ich ende
• du endest
• er/sie/es endet
• wir enden

Since der Hosenbund is singular, the correct form is:

• Der Hosenbund endet ...

endet means ends or finishes.

In this sentence, it describes where the waistband comes to an end. In sewing, clothing, or pattern-making language, this is a natural way to describe how a piece of fabric or garment construction is positioned.

So Der Hosenbund endet an der Seitennaht means that the waistband ends at the side seam.

The preposition an can take either:

• dative for a location
• accusative for direction or movement toward something

Here, the sentence describes a location/endpoint, not movement from one place to another. So German uses the dative:

• an der Seitennaht = at the side seam

That is why you get der here: it is the dative singular form of the feminine article die.

Because these are two different grammatical forms of der.

1. Der Hosenbund

   • Hosenbund is masculine
   • this der is nominative masculine singular

2. an der Seitennaht

   • Seitennaht is feminine
   • this der is dative feminine singular

So the two der forms look the same, but they do different jobs.

Seitennaht is another compound noun:

• Seite = side
• Naht = seam

Again, the last part decides the gender.
Since die Naht is feminine, the whole compound is feminine:

• die Seitennaht = side seam

And after an in this sentence, it becomes dative:

• an der Seitennaht

Literally, it breaks down as:

• Seite = side
• Naht = seam

So Seitennaht literally means side seam.

This is a very common kind of German compound: instead of using several separate words, German often joins them into one noun.

Usually an der Seitennaht is the most natural choice here.

Why?

• an often works well for something that is at, on, or along an edge, line, or boundary
• a seam is like a line on the garment, so an der Seitennaht sounds natural

Other prepositions would change the nuance:

• bei der Seitennaht = near the side seam
• auf der Seitennaht = on top of the side seam, which sounds less natural in this context

So endet an der Seitennaht is the best fit.

No, this is a very normal German main clause:

• Der Hosenbund = subject
• endet = conjugated verb
• an der Seitennaht = prepositional phrase

German main clauses usually put the conjugated verb in second position:

• Der Hosenbund | endet | an der Seitennaht

So this sentence follows standard word order.

Not really in casual conversation, but it is perfectly normal in a sewing, tailoring, fashion, or garment-construction context.

A native speaker would understand it, especially in:

• sewing instructions
• pattern descriptions
• clothing specifications
• fashion design notes

So the grammar is completely standard, even if the topic is somewhat technical.