Während die Suppe auf dem Herd köchelt, rühre ich sie langsam um.

Because während is a subordinating conjunction here. In a subordinate clause, the conjugated verb usually goes to the end.

So:

• Während = while
• die Suppe = subject
• auf dem Herd = on the stove
• köchelt = is simmering / simmers

That gives the pattern:

• Während ... köchelt

This is very typical in German:

• Während ich warte, lese ich.
• Weil es regnet, bleibe ich zu Hause.

In both cases, the verb in the subordinate clause goes to the end.

German main clauses follow the verb-second rule. The conjugated verb must be in the second position.

Here, the whole subordinate clause comes first:

• Während die Suppe auf dem Herd köchelt, = position 1
• rühre = position 2
• ich = comes after the verb

So you get:

• Während die Suppe auf dem Herd köchelt, rühre ich sie langsam um.

If you started with the main clause, you would say:

• Ich rühre sie langsam um, während die Suppe auf dem Herd köchelt.

Then ich is position 1 and rühre is still position 2.

In German, subordinate clauses are separated from main clauses by commas.

Since Während die Suppe auf dem Herd köchelt is a subordinate clause, it must be followed by a comma before the main clause:

• Während die Suppe auf dem Herd köchelt, rühre ich sie langsam um.

This is stricter than in English, where commas can sometimes be more flexible.

Here, während is a conjunction meaning while.

It connects two actions happening at the same time:

• the soup is simmering
• I am stirring it slowly

Important: während can also be a preposition in other sentences, and then it takes the genitive, for example:

• während des Essens = during the meal

But in your sentence, it is clearly a conjunction because it introduces a whole clause:

• Während die Suppe ... köchelt

köcheln means to simmer or to cook gently at a low bubble.

That is different from kochen, which can mean:

• to cook
• to boil

So:

• Die Suppe kocht. = The soup is boiling / cooking.
• Die Suppe köchelt. = The soup is simmering gently.

In a cooking context, köcheln suggests a softer, slower heat than kochen.

Because this sentence describes a location, not movement.

German uses:

• dative for location: wo? = where?
• accusative for movement toward a destination: wohin? = to where?

So:

• auf dem Herd = on the stove → location, dative
• auf den Herd = onto the stove → movement, accusative

In your sentence, the soup is already there, so it is:

• auf dem Herd

Because Suppe is a feminine noun in German:

• die Suppe

So the pronoun referring to it is:

• sie = it (for feminine nouns)

This can feel strange for English speakers because English uses it for things, while German pronouns match the noun’s grammatical gender.

So:

• die Suppe → sie
• der Löffel → er
• das Messer → es

In this sentence, sie means it, not she.

Because umrühren is a separable verb.

Its full infinitive is:

• umrühren = to stir

In a normal main clause, separable verbs split apart:

• the conjugated part goes in the usual verb position
• the prefix goes to the end

So:

• ich rühre ... um

That is why the sentence says:

• rühre ich sie langsam um

More examples:

• Ich mache die Tür auf. (aufmachen)
• Er steht früh auf. (aufstehen)

But in the infinitive or in subordinate clauses, the verb stays together:

• Ich will die Suppe umrühren.
• ..., weil ich die Suppe umrühre.

langsam is an adverb meaning slowly, and German word order gives some flexibility, but some placements sound more natural than others.

In your sentence:

• rühre ich sie langsam um

This is a very natural placement.

You may also hear:

• rühre ich langsam die Suppe um
• rühre ich die Suppe langsam um

But with the pronoun sie, rühre ich sie langsam um sounds especially normal.

A useful general idea:

• short pronouns like sie often come earlier
• adverbs like langsam often come before the final verb part or prefix

Yes. That is completely correct.

Both versions are grammatical:

• Während die Suppe auf dem Herd köchelt, rühre ich sie langsam um.
• Ich rühre sie langsam um, während die Suppe auf dem Herd köchelt.

The difference is mostly one of emphasis and style:

• starting with Während ... puts the time setting first
• starting with Ich ... puts the action first

German often moves elements around for emphasis, but the verb-position rules still stay the same.

Yes. rühre is present tense, and German often uses the simple present where English would use the present progressive.

So German says:

• Ich rühre die Suppe um.

English might say:

• I stir the soup
• I am stirring the soup

In context, German present tense can easily mean I am stirring it right now.

The same is true for:

• die Suppe köchelt = the soup simmers / is simmering

Yes, when während is used as a conjunction, it introduces a subordinate clause, so the verb goes to the end.

Examples:

• Während ich esse, lese ich.
• Während er arbeitet, hört er Musik.

But remember: während can also be a preposition, and then there is no clause after it:

• während des Films = during the film
• während der Arbeit = during work

So:

• während + clause → verb goes to the end
• während + noun phrase → no verb-final clause, because it is not a clause at all

Just like English, German often uses a pronoun to avoid repetition once the thing has already been identified.

So instead of saying:

• Während die Suppe auf dem Herd köchelt, rühre ich die Suppe langsam um.

German naturally says:

• Während die Suppe auf dem Herd köchelt, rühre ich sie langsam um.

This sounds smoother and more natural. Since die Suppe has just been mentioned, sie is clear and easy to understand.