Der Abnäher endet am Hosenbund.

Abnäher is a masculine noun, so in the nominative singular it takes der: der Abnäher.

In this sentence, der Abnäher is the subject, so nominative is required.

A useful detail: many German nouns ending in -er are masculine, although not always. So it is something you usually learn together with the article: der Abnäher.

In sewing and clothing language, Abnäher means a dart: a folded and stitched part of fabric used to shape a garment.

Even if the meaning has already been given to you, this is helpful because it is a somewhat specialized word and not a very common everyday noun for beginners.

The infinitive is enden = to end.

Here it is conjugated for third person singular because the subject is der Abnäher:

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

So Der Abnäher endet ... means The dart ends ...

Because it is the thing that is doing the action of ending.

In Der Abnäher endet am Hosenbund, the sentence structure is:

• Der Abnäher = subject
• endet = verb
• am Hosenbund = location/endpoint phrase

So the sentence literally works like The dart ends at the waistband.

Am is the contraction of an dem:

• an dem → am

So:

• am Hosenbund = at the waistband / on the waistband

This contraction is extremely common in German and you should usually expect it when an is followed by dem.

Because here an is used with the dative, not the accusative.

A very common rule in German is:

• dative after two-way prepositions for location
• accusative for direction toward something

In this sentence, the idea is that the dart ends at a certain place. It describes the endpoint as a location, so German uses:

• an dem Hosenbund → am Hosenbund

If the sentence were expressing motion toward the waistband in a different way, accusative might appear, but not here with endet am ....

Yes. Hosenbund is a compound noun:

• Hose = trousers / pants
• Bund = band, edge, waistband

Together, Hosenbund means waistband of trousers/pants.

The -n- in Hosen- comes from the form used in compounds. German often builds long nouns this way, and the last part determines the grammatical gender.

Because the final part of the compound is Bund, and Bund is masculine: der Bund.

In German compounds, the last element determines the gender, so:

• der Bund
• therefore der Hosenbund

That is also why the dative form here is dem Hosenbund, which becomes am Hosenbund.

Because German main clauses usually follow the verb-second rule.

In this sentence:

• 1st position: Der Abnäher
• 2nd position: endet
• then: am Hosenbund

So the basic structure is completely normal for a German statement.

You could also move another element to the front for emphasis, and the verb would still stay second:

• Am Hosenbund endet der Abnäher.

That sounds more marked or stylistic, but it is grammatically correct.

Yes, but the meaning or nuance would change slightly.

• am Hosenbund = at the waistband
  This is the wording in your sentence and suggests the dart ends at that boundary/area.

• im Hosenbund = in the waistband
  This would suggest the dart ends inside the waistband area.

• bis zum Hosenbund = up to the waistband
  This emphasizes the extent or limit reached.

So am Hosenbund is not arbitrary; it gives a specific spatial relationship.

Abnäher is pronounced approximately like AHP-nay-er.

A few points:

• Ab- sounds like ahp
• näh has a long ä sound, somewhat like the vowel in English air for many speakers, though not exactly the same
• -er at the end is the usual German -er ending

Also notice the umlaut ä in Näher. It is important in spelling and pronunciation.

Yes. The vocabulary is specialized because of Abnäher, but the grammar is very standard:

• Der Abnäher = nominative subject
• endet = present tense verb
• am Hosenbund = prepositional phrase with an + dative

So even if you never use sewing terms, the sentence is a good example of normal German sentence structure.