Die Eier scheinen sich mit der Milch gut zu vermischen, wenn wir langsam rühren.

Because die Eier is plural and means the eggs.

• das Ei = the egg (singular)
• die Eier = the eggs (plural)

Since the subject is plural, the verb must also be plural:

• das Ei scheint = the egg seems
• die Eier scheinen = the eggs seem

So scheinen matches the plural subject Eier.

This is a very common German structure with scheinen.

German often uses:

• scheinen + zu + infinitive

So:

• Die Eier scheinen sich mit der Milch gut zu vermischen.

literally follows the pattern:

• The eggs seem to mix well with the milk.

Here:

• scheinen is the conjugated verb
• zu vermischen is the infinitive construction that goes toward the end of the clause

This is normal German word order. A few similar examples:

• Er scheint müde zu sein. = He seems to be tired.
• Das Kind scheint tief zu schlafen. = The child seems to be sleeping deeply.

Because sich vermischen is used here in a reflexive way.

In this sentence, sich vermischen means something like:

• to mix together
• to become mixed
• to blend with each other / with something else

So Die Eier scheinen sich mit der Milch gut zu vermischen suggests that the eggs themselves are blending well with the milk.

This is slightly different from a more direct transitive structure such as:

• Wir vermischen die Eier mit der Milch.
  = We mix the eggs with the milk.

So:

• etwas vermischen = to mix something
• sich vermischen (mit) = to mix/blend together, to become mixed with

Because the preposition mit always takes the dative case.

The noun Milch is feminine:

• nominative: die Milch
• dative: der Milch

So after mit, you must say:

• mit der Milch = with the milk

This is one of the most important preposition-case combinations to memorize:

• mit + dative

Other examples:

• mit dem Löffel = with the spoon
• mit den Händen = with the hands

Both relate to mixing, but vermischen often sounds more like mix together thoroughly or blend together.

A rough distinction:

• mischen = mix
• vermischen = mix together, combine, blend

In cooking contexts, vermischen can sound a bit more natural when two ingredients are combining into one mixture.

Examples:

• Ich mische den Teig. = I mix the dough.
• Ich vermische Mehl und Zucker. = I mix flour and sugar together.

In real usage, there can be overlap, but vermischen often emphasizes the combining of substances.

Because gut is an adverb here, describing how the eggs mix with the milk.

So:

• sich gut vermischen = to mix well

In German, adverbs like gut, langsam, schnell, leicht often appear before the infinitive or participle they modify.

Compare:

• Das lässt sich gut erklären. = That can be explained well.
• Die Zutaten lassen sich leicht vermischen. = The ingredients can be mixed easily.

So gut tells us the quality of the mixing.

Here wenn means when in the sense of whenever / when we do this.

So:

• wenn wir langsam rühren = when we stir slowly

In many contexts, wenn can mean either if or when, depending on the sentence.

Here the meaning is more like a general condition or repeated situation:

• The eggs seem to mix well with the milk when we stir slowly.

That is why English might also understand it almost like:

• if we stir slowly

But in this sentence, when is usually the most natural translation.

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

Main clause:

• Wir rühren langsam.

Subordinate clause with wenn:

• ..., wenn wir langsam rühren.

That is a basic German rule:

• in subordinate clauses, the finite verb goes to the end

Other examples:

• ..., weil er müde ist.
• ..., dass sie heute kommt.
• ..., wenn wir Zeit haben.

So rühren moves to the end because of wenn.

In German, as in English, sometimes the object can be left unstated if it is obvious from context.

Here, rühren means to stir, and the listener naturally understands that we are stirring the mixture involving the eggs and milk.

So wenn wir langsam rühren is perfectly normal, even without saying exactly what is being stirred.

You could make it more explicit:

• ..., wenn wir die Mischung langsam rühren.
• ..., wenn wir Eier und Milch langsam rühren.

But that is not necessary if the context already makes it clear.

Yes, but it would change the structure and slightly change the perspective.

With sich:

• Die Eier scheinen sich mit der Milch gut zu vermischen.
• The eggs seem to blend well with the milk.

Without sich, you would normally need a direct object:

• Wir scheinen die Eier gut mit der Milch zu vermischen.
• We seem to mix the eggs well with the milk.

So:

• sich vermischen focuses on the ingredients blending
• etwas vermischen focuses on someone mixing something

Both are possible in German, but they are not identical structures.

Yes, somewhat.

scheinen means to seem, so the sentence does not sound as direct or absolute as simply saying that the eggs mix well.

Compare:

• Die Eier vermischen sich gut mit der Milch.
  = The eggs mix well with the milk.

• Die Eier scheinen sich gut mit der Milch zu vermischen.
  = The eggs seem to mix well with the milk.

Using scheinen can suggest:

• an observation
• a cautious conclusion
• something that appears to be true

So it adds a layer of tentativeness.

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

Here the main clause is:

• Die Eier scheinen sich mit der Milch gut zu vermischen

And the subordinate clause is:

• wenn wir langsam rühren

German punctuation requires the comma before the subordinate clause:

• ..., wenn wir langsam rühren.

This is a very important rule in German, much stricter than in English.