Wenn die Milch aufkocht, stelle ich die Hitze runter und lasse die Suppe noch zehn Minuten köcheln.

Because wenn introduces a subordinate clause, and in German subordinate clauses the conjugated verb goes to the end.

So:

• main clause: Die Milch kocht auf.
• subordinate clause: wenn die Milch aufkocht

This is also a nice example of a separable verb. The verb is aufkochen. In a normal main clause, it splits into kocht ... auf. In a subordinate clause, it stays together as aufkocht at the end.

German main clauses usually put the conjugated verb in second position.

Here, the whole wenn-clause comes first:

• Wenn die Milch aufkocht, = position 1

That means the verb of the main clause must come immediately after it:

• stelle ich die Hitze runter

So the structure is:

• [subordinate clause] + [verb] + [subject] + ...

This is very common in German:

• Wenn ich Zeit habe, komme ich vorbei.
• Nach dem Essen gehe ich nach Hause.

Here wenn means when.

German usually uses:

• wenn for repeated situations, general situations, or present/future situations
• als for a single completed event in the past

In this sentence, it sounds like a general cooking instruction or a repeated situation, so wenn is the natural choice.

Examples:

• Wenn die Milch aufkocht, stelle ich die Hitze runter.
  = whenever / when the milk comes to a boil, I turn the heat down

• Als die Milch aufkochte, stellte ich die Hitze runter.
  = when the milk came to a boil (one specific past event), I turned the heat down

Aufkochen means something like to come to a boil, to boil up, or to start boiling strongly.

With milk, this is especially natural, because milk can suddenly rise when it reaches boiling point.

So Wenn die Milch aufkocht suggests:

• the milk reaches the point of boiling
• possibly it starts rising up in the pot

It is a bit more specific than just kochen.

Yes, you could, but the nuance is slightly different.

• Wenn die Milch kocht = when the milk is boiling
• Wenn die Milch aufkocht = when the milk comes to a boil / boils up

So aufkocht often focuses more on the moment the boiling starts or becomes noticeable. In cooking instructions, that can be useful, especially with milk.

Die Hitze runterstellen means to turn the heat down.

The word runter is a colloquial form of herunter. Both are common, but runter sounds more everyday and conversational.

So these mean basically the same thing:

• die Hitze runterstellen
• die Hitze herunterstellen

The verb is separable:

• infinitive: runterstellen
• main clause: Ich stelle die Hitze runter.
• subordinate clause: ..., weil ich die Hitze runterstelle.

This is an idiomatic way to talk about lowering the heat on the stove.

This uses lassen + accusative object + infinitive.

Pattern:

• Ich lasse die Suppe köcheln.

This means:

• I let the soup simmer
• or more naturally in English, I leave the soup to simmer / I keep the soup simmering

Important points:

1. die Suppe is the object of lassen
2. köcheln stays in the infinitive
3. there is no zu after lassen

So:

• Ich lasse die Suppe köcheln. ✅
• Ich lasse die Suppe zu köcheln. ❌

They are related, but not the same.

• kochen = to boil / to cook
• köcheln = to simmer gently

So köcheln suggests low heat and gentle bubbling, not a full strong boil.

That fits the sentence well:

1. the milk comes to a boil
2. the heat gets turned down
3. the soup simmers gently for ten more minutes

Here noch means for another or ten more minutes.

So:

• noch zehn Minuten köcheln = simmer for another ten minutes

This noch does not mean still in the usual English sense. It adds the idea of extra time continuing from now.

Compare:

• Warte noch fünf Minuten. = Wait another five minutes.
• Die Suppe kocht noch. = The soup is still boiling.

Same word, but different nuance depending on context.

No. They have different grammatical roles, but because they are all feminine singular nouns, the form die appears in both nominative and accusative.

Their roles are:

• die Milch = subject of aufkocht
• die Hitze = direct object of stelle ... runter
• die Suppe = direct object of lasse

So even though the form looks the same, the function changes.

That can be confusing for learners, because with feminine singular nouns:

• nominative = die
• accusative = die

You only see the difference from the sentence structure, not from the article form.

It is perfectly normal everyday German. The most conversational element is runter instead of herunter.

So:

• stelle ich die Hitze runter = everyday, natural
• stelle ich die Hitze herunter = slightly more formal or neutral

Everything else is standard and common, especially in recipes or spoken instructions.