Wir nähen die Seitennaht um.

Because the verb here is umnähen, which is a separable verb. In a normal main clause, the conjugated part goes in the usual verb position, and the prefix moves to the end:

• Wir nähen ... um.

So nähen is the conjugated part, and um is the separated prefix.

Compare:

• Wir nähen die Seitennaht um.
• ..., weil wir die Seitennaht umnähen.

In the second example, the verb is at the end of a subordinate clause, so the prefix stays attached.

No. Here, um is not a preposition like in um den Tisch (around the table). It is the prefix of the separable verb umnähen.

So this sentence is built from:

• umnähen = the full verb
• wir nähen ... um = the split form in a main clause

That is why um does not introduce a noun phrase here.

The basic form is umnähen.

The sentence uses the wir form in the present tense:

• ich nähe um
• du nähst um
• er/sie/es näht um
• wir nähen um
• ihr näht um
• sie/Sie nähen um

So nähen matches wir.

Because Seitennaht is a feminine noun: die Seitennaht.

In this sentence it is also the direct object, and in the singular, the feminine article stays die in both nominative and accusative:

• nominative: die Seitennaht
• accusative: die Seitennaht

So the form does not change, even though its role in the sentence is different.

Yes. German very often combines nouns into one compound word.

Here:

• Seite = side
• Naht = seam

Together:

• die Seitennaht = side seam

The last part, Naht, is the main noun, so it determines the gender. Since Naht is feminine, Seitennaht is also feminine.

Because die Seitennaht is the direct object of the verb.

In English, some actions also take a direct object without a preposition, and German often does the same. Here the structure is simply:

• Wir = subject
• nähen ... um = verb
• die Seitennaht = object

So no extra preposition is needed.

In standard German main-clause word order, no. That sounds wrong because umnähen is separable.

In a main clause, you normally must split it:

• Wir nähen die Seitennaht um.

You keep it together in forms such as:

• infinitive: die Seitennaht umnähen
• subordinate clause: ..., weil wir die Seitennaht umnähen
• past participle: umgenäht

The past participle of umnähen is umgenäht.

So you can say:

• Wir haben die Seitennaht umgenäht.

That is typical for separable verbs: the prefix stays attached to the participle, and ge usually comes between the prefix and the verb stem.

It changes the meaning from plain sew (nähen) to a more specific action: resew / alter by sewing / sew differently again, depending on context.

So:

• nähen = to sew
• umnähen = to resew, alter, or sew again in a different way

The exact English wording can vary by context, but grammatically the important point is that um- is part of the verb.

Yes, German can move elements for emphasis, as long as the verb-second rule is respected.

For example:

• Die Seitennaht nähen wir um.

This is possible, but it sounds more marked and emphasizes die Seitennaht. The neutral version is:

• Wir nähen die Seitennaht um.

So learners should usually treat the original sentence as the normal default order.

Because wir is the pronoun meaning we, and the verb form nähen agrees with it.

So the sentence structure is:

• Wir = subject
• nähen ... um = verb
• die Seitennaht = object

German often relies on both position and verb agreement to make this clear.

Yes. Wir nähen die Seitennaht um is in the present tense.

Depending on context, German present tense can mean:

• We resew the side seam
• We are resewing the side seam
• sometimes even a near future meaning, like We’re going to resew the side seam

German uses the present tense more broadly than English does.