Während wir das Hosenbein abstecken, markiert er den Saum.

Während is a subordinating conjunction meaning while in this sentence. It introduces a subordinate clause: Während wir das Hosenbein abstecken.

Because it is a subordinating conjunction, it changes the word order in that clause: the conjugated verb goes to the end.

Because während creates a subordinate clause, German sends the finite verb to the end of that clause.

So instead of a main-clause pattern like:

Wir stecken das Hosenbein ab.

you get:

Während wir das Hosenbein abstecken ...

Here, abstecken appears as one full verb form at the end. In a subordinate clause, separable-prefix verbs are usually written together rather than split apart.

This is because the whole während-clause comes first, and in German the finite verb of the main clause must come in the second position.

The first position is occupied by the entire opening clause:

Während wir das Hosenbein abstecken

So the main clause begins with the verb:

markiert er den Saum

That is normal German word order. If the main clause came first, you would say:

Er markiert den Saum, während wir das Hosenbein abstecken.

This is about gender and case.

Both nouns are direct objects here, so they are in the accusative case.

• das Hosenbein is neuter

  • nominative: das Hosenbein
  • accusative: das Hosenbein
    Neuter articles do not change between nominative and accusative singular.

• der Saum is masculine

  • nominative: der Saum
  • accusative: den Saum
    Masculine singular articles do change in the accusative.

So:

• wir stecken das Hosenbein ab
• er markiert den Saum

In this context, abstecken is a tailoring/sewing verb. It usually means something like:

• to pin up
• to mark out
• to fit and pin
• to determine the correct length by pinning

With ein Hosenbein, it suggests adjusting or pinning the trouser leg to the right length before finishing it.

It is also a separable verb in main clauses:

• Wir stecken das Hosenbein ab.

But in subordinate clauses and infinitive forms, it stays together:

• ..., während wir das Hosenbein abstecken.

Yes, das Hosenbein is singular and means the trouser leg or pant leg.

German often uses a singular noun when talking about one specific part of a garment. So this sentence refers to one leg of the trousers, not both.

• das Hosenbein = one trouser leg
• die Hosenbeine = the trouser legs

In German, subordinate clauses are separated from the main clause with a comma.

Since Während wir das Hosenbein abstecken is a subordinate clause and markiert er den Saum is the main clause, a comma is required:

Während wir das Hosenbein abstecken, markiert er den Saum.

This is stricter than in English, where comma use can sometimes be more flexible.

The sentence is in the present tense.

• wir ... abstecken
• er markiert

In German, the present tense is often used for actions happening right now, so it can correspond to English are pinning up / is marking as well as simple present forms.

So even if English might prefer:

• While we’re pinning up the trouser leg, he’s marking the hem

German naturally uses the present tense for that.

Yes. A very natural alternative is:

Er markiert den Saum, während wir das Hosenbein abstecken.

The basic meaning stays the same. The difference is mostly one of emphasis and sentence flow.

• Starting with Während ... emphasizes the simultaneous background action first.
• Starting with Er markiert ... puts the focus first on what he is doing.

Der Saum means the hem — the finished edge of a piece of clothing, especially at the bottom.

In this tailoring context:

• das Hosenbein = the trouser leg
• der Saum = the hem of that leg

So the sentence describes two related tailoring actions: one person is pinning/adjusting the trouser leg, and another is marking the hem.

Yes, when während is used as a conjunction meaning while, it introduces a subordinate clause, so the verb goes to the end.

Examples:

• Während ich koche, hört sie Musik.
• Während wir warten, liest er ein Buch.
• Während wir das Hosenbein abstecken, markiert er den Saum.

So the structure is very consistent and worth learning as a pattern.