Wir prüfen das Knopfloch, bevor wir die Innennaht schließen.

These articles show the grammatical gender of the nouns:

• das Knopfloch = the buttonhole
  
  • Knopfloch is neuter
• die Innennaht = the inner seam / inside seam
  
  • Innennaht is feminine

In this sentence, both nouns are in the accusative, but:

• for neuter nouns, das stays das
• for feminine nouns, die stays die

So the forms happen to look the same as the nominative here.

Because it is the direct object of prüfen.

• Wir = subject, the people doing the action
• prüfen = check / inspect
• das Knopfloch = the thing being checked

So the structure is:

• Wir prüfen das Knopfloch.
• We check the buttonhole.

That makes das Knopfloch the accusative object.

Because bevor introduces a subordinate clause, and in German subordinate clauses the conjugated verb usually goes to the end.

So:

• main clause: Wir prüfen das Knopfloch
• subordinate clause: bevor wir die Innennaht schließen

That final position of schließen is completely normal after bevor.

Because German normally uses a comma to separate a main clause from a subordinate clause.

So in:

• Wir prüfen das Knopfloch, bevor wir die Innennaht schließen.

the comma is required, not optional.

This is different from English, where punctuation can sometimes be more flexible.

Because German normally states the subject in each clause.

So even though both clauses have the same subject, German still says:

• Wir prüfen ...
• bevor wir ... schließen

You cannot normally leave out the second wir here.

bevor means before as a conjunction. It introduces a whole clause:

• bevor wir die Innennaht schließen
  = before we close the inner seam

vor is usually a preposition, so it is followed by a noun phrase, not a full clause:

• vor dem Schließen der Innennaht
  = before closing the inner seam

So:

• bevor
  • clause
• vor
  • noun phrase

Yes, here prüfen means something like:

• check
• inspect
• examine

In a sewing or manufacturing context, it suggests making sure the buttonhole is correct before continuing.

It can sometimes sound a bit more careful or technical than everyday English check.

Yes. schließen literally means to close, but in this context it means something like:

• close up
• sew shut
• finish by closing

With a seam, German often uses schließen where English might say sew closed or close up.

So die Innennaht schließen means closing that seam as part of the sewing process.

Innennaht is a compound noun, which is very common in German.

It is made from:

• innen = inside / inner
• Naht = seam

Together:

• Innennaht = inner seam / inside seam

German usually writes such combinations as one word, not two.

Also, the grammatical gender of a compound noun is determined by the last part:

• die Naht → therefore die Innennaht

Because it is another German compound noun:

• Knopf = button
• Loch = hole

So:

• Knopfloch = buttonhole

German very often combines related nouns into a single word. English sometimes does this too, but less consistently.

No. In this sentence they are conjugated:

• wir prüfen
• wir schließen

The infinitive forms are:

• prüfen
• schließen

But because the subject is wir, the present-tense forms are:

• wir prüfen
• wir schließen

The reason schließen looks like it might still be an infinitive is that the wir form in the present tense is identical to the infinitive for most verbs.

The base verb is schließen, spelled with ie and ß.

Important forms are:

• infinitive: schließen
• ich schließe
• du schließt
• wir schließen
• past participle: geschlossen

The ie is just part of the verb’s spelling. It is not added because of the sentence structure.

Yes. German can also say:

• Bevor wir die Innennaht schließen, prüfen wir das Knopfloch.

That means the same thing.

When the subordinate clause comes first, the main clause still follows the normal German rule that the finite verb comes second, so you get:

• prüfen wir not
• wir prüfen

Because with neuter singular and feminine singular, the accusative article often looks the same as the nominative:

• nominative: das Knopfloch, die Innennaht
• accusative: das Knopfloch, die Innennaht

So even though they are objects here, the articles do not visibly change.

You would see a clearer change with a masculine noun, for example:

• nominative: der Stoff
• accusative: den Stoff

Yes. It is in the present tense:

• Wir prüfen ...
• wir ... schließen

German often uses the present tense for:

• actions happening now
• general habits
• instructions in a process
• steps in a procedure

So this sentence can naturally describe a standard production or sewing step, not just something happening at this exact moment.