Wir trennen die Naht auf und prüfen die Passform.

Because auftrennen is a separable verb in German.

The full verb is auftrennen. In a normal main clause, German separates it:

• Wir trennen die Naht auf.

So:

• trennen = the conjugated part, which goes in the usual verb position
• auf = the separable prefix, which moves to the end of the clause

This is very common in German:

• Ich mache die Tür auf. = aufmachen
• Wir probieren das Kleid an. = anprobieren

The full verb is auftrennen.

• trennen by itself means to separate
• auftrennen means something more specific, often to unpick, to rip open, or to undo stitching

So in this sentence, the meaning is not just general separation. It refers specifically to opening or undoing the seam.

Because both nouns are feminine singular nouns:

• die Naht = seam
• die Passform = fit

In German, every noun has a grammatical gender, and these two happen to be feminine.

Also, in this sentence both are in the accusative singular, and for feminine nouns the article stays die:

• nominative: die Naht
• accusative: die Naht

The same is true for die Passform.

They are both in the accusative case because they are the direct objects of the verbs:

• Wir trennen die Naht auf.
• Wir prüfen die Passform.

A simple way to think about it:

• What do we undo? die Naht
• What do we check? die Passform

Those are the things receiving the action, so they are direct objects.

Because the subject wir applies to both verbs.

German often avoids repeating the subject when two verbs are joined by und and have the same subject:

• Wir trennen die Naht auf und prüfen die Passform.

This works just like in English:

• We undo the seam and check the fit.

You could repeat wir, but it would usually sound unnecessary here:

• Wir trennen die Naht auf und wir prüfen die Passform.

That is grammatical, but less natural in this context.

Because prüfen is an ordinary, non-separable verb, while auftrennen is separable.

So the sentence contains:

• trennen ... auf from auftrennen
• prüfen as a normal verb

Each part follows its own verb pattern:

1. Wir trennen die Naht auf
2. und prüfen die Passform

Only the separable verb has a prefix that moves to the end.

Yes, it is in the present tense:

• Wir trennen ... auf
• wir prüfen ...

But in German, the present tense can also be used in contexts like:

• describing a process
• giving step-by-step instructions
• explaining what is done in a workshop or fitting

So depending on context, it might mean:

• We undo the seam and check the fit or
• something like Next, we undo the seam and check the fit

No, not in a normal main clause.

In a main clause, separable verbs split:

• Wir trennen die Naht auf.

You keep the full infinitive auftrennen only in certain structures, such as:

• Wir wollen die Naht auftrennen.
• Es ist wichtig, die Naht aufzutrennen.

So:

• main clause: Wir trennen ... auf
• infinitive form: auftrennen

Because German normally does not use a comma before und when it simply links two equal parts of a sentence.

Here, und connects two verbs with the same subject:

• Wir trennen die Naht auf und prüfen die Passform.

So no comma is needed.

Not exactly.

Passform means fit: how well a garment matches the shape of the body. It is about whether something sits correctly, comfortably, or as intended.

Größe means size.

So:

• die Größe = the numerical/labeled size
• die Passform = how the item actually fits

A garment can be the right Größe but still have a poor Passform.

The infinitive forms are:

• auftrennen
• prüfen

So the sentence could be reduced to the dictionary forms as:

• die Naht auftrennen
• die Passform prüfen

This is useful when learning vocabulary, because in the actual sentence the separable verb is split apart:

• dictionary form: auftrennen
• sentence form: trennen ... auf