Während ich die Nähmaschine prüfe, wechselt sie die Garnrolle.

Because Während introduces a subordinate clause. In German, the finite verb usually goes to the end of a subordinate clause.

So:

• Während ich die Nähmaschine prüfe = While I check/am checking the sewing machine

Structure:

• Während = while
• ich = I
• die Nähmaschine = the sewing machine
• prüfe = check / am checking

That final verb position is one of the most important patterns in German subordinate clauses.

Here, Während means while and functions as a subordinating conjunction.

It links two actions that happen at the same time:

• Während ich die Nähmaschine prüfe
• wechselt sie die Garnrolle

So the sentence means that one action happens during the other.

A useful note: während can also be a preposition in other sentences, but here it is clearly a conjunction because it introduces a full clause with its own subject and verb: ich ... prüfe.

Because the sentence begins with a subordinate clause, and that whole clause takes up the first position in the main clause structure.

German main clauses usually follow the verb-second rule:

• Position 1: Während ich die Nähmaschine prüfe
• Position 2: wechselt
• Then: sie die Garnrolle

So the verb must come immediately after the opening subordinate clause.

This is often called inversion in English-language explanations.

Compare:

• Sie wechselt die Garnrolle.
• Während ich die Nähmaschine prüfe, wechselt sie die Garnrolle.

Both are correct; the word order changes because of what comes first.

Because prüfen is the infinitive, and here you need the conjugated form to match ich.

Present tense of prüfen:

• ich prüfe
• du prüfst
• er/sie/es prüft
• wir prüfen
• ihr prüft
• sie/Sie prüfen

So:

• ich prüfe = I check / I am checking

The same thing happens with wechselt:

• infinitive: wechseln
• sie wechselt = she changes / is changing

No. Both die Nähmaschine and die Garnrolle are direct objects here.

They have die because they are feminine singular nouns, and for feminine singular, the definite article is die in both the nominative and the accusative.

So you have to determine their role from the sentence structure and the verb, not just from the article.

In this sentence:

• ich is the subject of prüfe
• die Nähmaschine is the thing being checked → direct object
• sie is the subject of wechselt
• die Garnrolle is the thing being changed/replaced → direct object

Grammatically, sie can mean:

• she
• it for a feminine noun
• they in other contexts

In this sentence, sie is 3rd person singular, so it could theoretically mean either she or it, depending on context.

Since die Nähmaschine is feminine, sie could grammatically refer to the sewing machine. But in most real contexts, the meaning shown to the learner probably makes clear whether sie means a woman or refers back to a feminine noun.

So the key point is:

• sie is grammatically flexible
• context tells you whether it means she or it

German often uses the simple present where English might prefer either:

• I check
• I am checking

So:

• ich prüfe can mean I check or I am checking
• sie wechselt can mean she changes or she is changing

With Während, English usually sounds more natural with the progressive:

• While I am checking the sewing machine, she is changing the spool

But German does not need a special progressive form here. The normal present tense is enough.

Because in German, a subordinate clause is normally separated from the main clause by a comma.

So the comma marks the boundary between:

• Während ich die Nähmaschine prüfe
• wechselt sie die Garnrolle

This is a very standard German punctuation rule and is much more consistent than in English.

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

In this sentence:

• sie = the one doing the action
• wechselt = changes/replaces
• die Garnrolle = the thing being changed/replaced

So die Garnrolle receives the action.

A useful vocabulary note: die Garnrolle means something like spool of thread or thread spool, depending on context.

It is also in the accusative case because it is the direct object of prüfen.

• ich = subject
• prüfe = check / inspect
• die Nähmaschine = the thing being checked

So:

• die Nähmaschine = accusative object of prüfe
• die Garnrolle = accusative object of wechselt

In this sentence:

• prüfen means to check, inspect, or test
• wechseln means to change, replace, or switch

So:

• die Nähmaschine prüfen = to inspect/check the sewing machine
• die Garnrolle wechseln = to change/replace the spool of thread

The most natural English translation depends on context, but those are the core meanings.

Yes. You can switch the order:

• Sie wechselt die Garnrolle, während ich die Nähmaschine prüfe.

That means the same thing.

The difference is mainly one of focus and style:

• starting with Während ich ... emphasizes the time setting first
• starting with Sie wechselt ... emphasizes the main action first

In both versions, the subordinate clause introduced by während still sends its verb to the end.

Yes. Nähmaschine is a compound noun:

• nähen = to sew
• Maschine = machine

So Nähmaschine literally means sewing machine.

German very often combines nouns into one word, and English learners need to get used to that pattern. The same idea appears in many other words, such as:

• Waschmaschine = washing machine
• Kaffeemaschine = coffee machine

So Nähmaschine is completely normal German word formation.