Als de saus te dik is, voegt Sofie wat water toe en roert zij rustig verder.

Because als introduces a subordinate clause.

In Dutch subordinate clauses, the finite verb usually goes to the end. Compare:

• Main clause: De saus is te dik.
• Subordinate clause: Als de saus te dik is

So this is normal Dutch word order after als.

Because the sentence starts with Als de saus te dik is, and that whole clause takes the first position in the sentence.

Dutch main clauses usually follow the verb-second rule: the finite verb must come in the second slot. So after the opening als clause, the next thing must be the finite verb:

• Als de saus te dik is, voegt Sofie ...

That is why the subject Sofie comes after voegt.

The verb is toevoegen, which is a separable verb.

In main clauses, separable verbs split:

• Sofie voegt wat water toe.

But in infinitives, participles, and many subordinate clauses, they stay together:

• water toevoegen
• toegevoegd
• dat Sofie wat water toevoegt

So voegt ... toe is exactly what you expect in a main clause.

This is another case of inversion.

The opening clause Als de saus te dik is applies to both actions: Sofie adds water, and she continues stirring. Dutch often keeps the inverted order in the second coordinated clause as well:

• voegt Sofie ... toe en roert zij ... verder

It is a neat, standard written structure.

Here wat means some.

So wat water means some water or a little water. It is very common with substances and uncountable nouns such as:

• wat melk
• wat suiker
• wat zout

It sounds natural and idiomatic in everyday Dutch.

Te means too.

So te dik means too thick, not just thick. It suggests that the sauce is thicker than desired.

Compare:

• dik = thick
• heel dik = very thick
• te dik = too thick

That little word te is important because it adds the idea of a problem.

No, not in the same way that toe is part of toevoegen.

Here the main verb is roeren, and verder is an adverb meaning something like on, further, or continue. So:

• roert rustig verder = continues stirring calmly / keeps stirring gently

So:

• toevoegen = separable verb
• verder roeren = ordinary verb + adverb

Because rustig is functioning as an adverb here, not as an adjective before a noun.

It describes how she stirs:

• zij roert rustig verder

Dutch adverbs often have the same basic form as the adjective stem, without -e.

Compare:

• de rustige kok = the calm cook
• zij roert rustig = she stirs calmly

Both zij and ze can mean she.

The difference is mainly style and stress:

• ze is more common in everyday spoken Dutch
• zij is a bit more formal, clearer, or more emphatic

In written Dutch, zij often appears where English learners might expect ze. In speech, many people would naturally say:

• ... en roert ze rustig verder

Yes, often you can.

Because the subject is still Sofie, Dutch can also coordinate the two verb phrases without repeating the subject:

• Als de saus te dik is, voegt Sofie wat water toe en roert rustig verder.

That is also natural Dutch. Repeating zij makes the structure a bit more explicit and slightly more formal in tone.

Because the sentence begins with a subordinate clause.

In standard Dutch, when a subordinate clause comes before the main clause, it is normally followed by a comma:

• Als de saus te dik is, voegt Sofie ...

There is no comma before en here, because the two coordinated parts are short and closely connected.