Wir stecken das Hosenbein ab.

Because the full verb is abstecken, which is a separable-prefix verb.

In a normal main clause, German usually puts:

• the conjugated part in position 2: stecken
• the prefix at the end: ab

So:

• Wir stecken das Hosenbein ab.

But in the infinitive, it stays together:

• abstecken

And in a subordinate clause, it also stays together:

• ..., weil wir das Hosenbein abstecken.

Because in a standard German main clause, the finite verb has to come in the second position.

Here, the finite verb is stecken, not abstecken as one single spoken unit in this sentence form. So the correct structure is:

• Wir = first element
• stecken = second-position verb
• das Hosenbein = object
• ab = separated prefix at the end

So Wir abstecken das Hosenbein is not correct standard word order.

In this context, abstecken is a tailoring/sewing word.

It usually means something like:

• to pin up
• to mark
• to measure out for shortening/alteration

So this is not just the everyday verb stecken meaning to put or to stick. In clothing contexts, abstecken often means marking where the fabric should be adjusted, usually with pins.

Das Hosenbein is a compound noun meaning trouser leg or pant leg.

It is made from:

• Hose = trousers / pants
• Bein = leg

Literally, it is something like pants-leg.

In clothing vocabulary, Bein can refer not only to a human leg, but also to the leg part of a garment.

Because the gender of a German compound noun is determined by the last part.

The last part here is Bein, and das Bein is neuter.

So:

• das Bein
• das Hosenbein

This is a very useful rule for German compounds.

That -n- is part of the standard compound form Hosenbein.

In German compounds, the first noun sometimes appears with a linking sound or a form that looks like a plural/base form. Unfortunately, this is not always fully predictable for learners.

So the practical answer is:

• the correct word is das Hosenbein
• it is best learned as a whole vocabulary item

In other words, Hosenbein is simply the established German word for trouser leg / pant leg.

Because it is the direct object of the verb abstecken.

The sentence is answering what are we pinning/marking? Answer: das Hosenbein

That makes it accusative.

With a neuter noun like das Hosenbein, the article is the same in nominative and accusative singular:

• nominative: das Hosenbein
• accusative: das Hosenbein

So you do not see a change in the article here, even though the case is accusative.

Yes, singular das Hosenbein refers to one trouser leg / pant leg.

So the sentence means that one leg is being pinned or marked.

If you wanted to say both pant legs, you could say:

• Wir stecken die Hosenbeine ab.

Sometimes German may also use die Hose if the whole garment is being altered, but das Hosenbein specifically focuses on one leg.

It is present tense.

• wir stecken

In German, the present tense can often cover both:

• we pin/mark the trouser leg
• we are pinning/marking the trouser leg

So depending on context, English might translate it with either a simple present or a present progressive form.