Die Seitennaht ist offen; wir trennen die Innennaht auf.

Both are compound nouns, which are very common in German.

• die Seite = side
• die Naht = seam

So:

• die Seitennaht = side seam
• die Innennaht = inner seam / inseam, depending on context

In German, the last part of the compound is the main noun, so both words are types of Naht. That also determines the grammatical gender.

Also note the linking -n- in Seitennaht. That extra sound often appears in compounds and is just part of how the word is formed.

Because the main noun in both compounds is die Naht, which is feminine.

In German compounds, the gender comes from the final element:

• die Naht → die Seitennaht
• die Naht → die Innennaht

So even though Seite and innen contribute to the meaning, Naht is what controls the gender.

Offen is an adjective meaning open.

So:

• Die Seitennaht ist offen = The side seam is open.

This describes the state or condition of the seam.

Geöffnet is the past participle of öffnen and would mean opened. That can also describe a state, but here offen sounds more natural because the focus is simply on the seam being open, not on the action that opened it.

So:

• offen = open
• geöffnet = opened

Because offen is not placed before a noun here. It is a predicate adjective, used after sein.

Compare:

• die offene Seitennaht = the open side seam
  → adjective before the noun, so it takes an ending
• Die Seitennaht ist offen = the side seam is open
  → adjective after ist, so no ending

This is the same basic idea as in English: The seam is open, not The seam is openedly or anything marked.

Auftrennen is a separable verb often used in sewing. It means something like:

• to unpick
• to rip open a seam
• to undo by separating stitches

It is more specific than just trennen.

Compare:

• trennen = to separate
• auftrennen = to unpick / rip open / separate by undoing stitches

So wir trennen die Innennaht auf means they are opening or undoing that seam by taking it apart.

Because auftrennen is a separable verb.

In a normal main clause, the verb splits:

• Wir trennen die Innennaht auf.

Here:

• trennen goes to the usual verb position
• auf goes to the end

This is a very common German pattern.

Other examples:

• Ich mache die Tür auf. = I open the door.
• Wir räumen das Zimmer auf. = We tidy up the room.

If the verb is in the infinitive, it stays together:

• Wir wollen die Innennaht auftrennen.

It is in the accusative case, because it is the direct object of trennen.

The thing being unpicked is die Innennaht, so that noun is the object.

For feminine singular nouns, the definite article is:

• nominative: die
• accusative: die

So the form looks the same, even though the function is different.

You can see the case change more clearly with a masculine noun:

• nominative: der Stoff
• accusative: den Stoff

But with die Naht, nominative and accusative are both die.

Yes, that can happen, especially in instructions, descriptions of a process, or technical contexts.

Wir trennen die Innennaht auf literally means we unpick the inner seam, but depending on the context it may function like:

• we unpick the inner seam
• the inner seam is then unpicked
• next, unpick the inner seam

German often uses wir in procedural language where English might use:

• an imperative
• passive
• a more impersonal style

So this can sound like a workshop instruction or a step in a sewing process.

Because it connects two closely related independent clauses:

• Die Seitennaht ist offen
• wir trennen die Innennaht auf

A semicolon shows a close relationship without using a conjunction like und.

You could also say:

• Die Seitennaht ist offen, und wir trennen die Innennaht auf.

The semicolon makes it feel a bit more formal, compact, or technical.

Yes, in the right context it can.

Literally, Innennaht is inner seam. In clothing contexts, especially with trousers, English often uses inseam.

So depending on what is being sewn or altered, translations might include:

• inner seam
• inside seam
• inseam

The exact English choice depends on the garment and situation.

Because all nouns are capitalized in German.

So here:

• die Seitennaht
• die Innennaht

are capitalized because they are nouns.

By contrast:

• offen is an adjective, so it is not capitalized
• wir is a pronoun, so it is not capitalized unless it begins a sentence

This is one of the most noticeable spelling features of German.

A very literal breakdown would be:

• Die Seitennaht = the side seam
• ist offen = is open
• wir trennen = we separate / we unpick
• die Innennaht = the inner seam
• auf = up / open, as part of the separable verb

So the overall meaning is not just we separate the inner seam up. The verb auftrennen has to be understood as a single unit.

A more natural English rendering would be something like:

• The side seam is open; we unpick the inner seam.
• The side seam is open; we rip open the inseam.

depending on the exact sewing context.