Anna zucht zacht, omdat de kinderen al in de gang staan te fluisteren.

Because omdat introduces a subordinate clause. In Dutch, subordinate clauses usually send the conjugated verb to the end.

So:

• main clause: Anna zucht zacht
• subordinate clause: omdat de kinderen al in de gang staan te fluisteren

Inside the omdat clause, the verb part comes at the end:

• staan te fluisteren

This is one of the most important Dutch word-order patterns to learn.

Staan te + infinitive is a very common Dutch construction. It means someone is standing and doing something, or more loosely busy doing something while standing.

So de kinderen staan te fluisteren means:

• the children are standing there whispering
• the children are whispering while standing

It often adds a sense of posture or visible ongoing activity.

If you said just de kinderen fluisteren, that would simply mean the children whisper / are whispering, without the extra idea that they are standing there doing it.

Here, te links the conjugated verb staan to the infinitive fluisteren.

This pattern appears in combinations like:

• zitten te lezen = to be sitting reading
• liggen te slapen = to be lying sleeping
• lopen te zingen = to be walking around singing
• staan te praten = to be standing talking

So in this sentence, te is part of the fixed structure staan te + infinitive.

They are similar, but not exactly the same.

• staan te fluisteren emphasizes that they are standing while whispering
• aan het fluisteren zijn emphasizes that the action is in progress

So:

• De kinderen staan te fluisteren = the children are standing there whispering
• De kinderen zijn aan het fluisteren = the children are whispering

In many contexts, both work, but staan te fluisteren gives a more physical picture.

In Dutch, plural nouns always take de, not het.

So:

• singular: het kind = the child
• plural: de kinderen = the children

This is a rule for all plural nouns.

Here al means already.

So de kinderen al in de gang staan te fluisteren means that the children are already standing in the hallway whispering.

It often suggests that this has happened sooner than expected, or that the speaker reacts to the fact that it is happening now.

Here zacht is functioning as an adverb, meaning softly or quietly.

Dutch often uses the same form for the adjective and the adverb:

• een zachte stem = a soft voice
• ze praat zacht = she speaks softly

So Anna zucht zacht means Anna sighs softly.

Yes, often you could.

• zacht = softly
• zachtjes = softly, quietly, a bit more gentle-sounding

In many everyday contexts, both are possible:

• Anna zucht zacht
• Anna zucht zachtjes

The version with zacht is perfectly natural here.

Because the subject is de kinderen, which is plural.

The infinitive is staan and the present-tense plural form is also staan:

• het kind staat = the child stands / is standing
• de kinderen staan = the children stand / are standing

So staan agrees with de kinderen.

In Dutch, a subordinate clause introduced by a conjunction like omdat is often separated by a comma, especially when it comes after the main clause.

So this structure is very normal:

• Anna zucht zacht, omdat ...

The comma helps show the boundary between the main clause and the reason clause.

In de gang means in the hallway.

Dutch uses in for being inside an enclosed space or area. A hallway is treated as a space you are in.

You may also hear op de gang in some contexts, especially when talking about being out in the corridor at school or outside a room. But in de gang is completely natural and clearly means in the hallway here.

It breaks down like this:

• Anna = subject
• zucht = finite verb
• zacht = adverb
• omdat = because
• de kinderen = subject of the subordinate clause
• al = already
• in de gang = location
• staan te fluisteren = are standing whispering

So the pattern is:

• main clause: Subject + verb + rest
• subordinate clause after omdat: subject + rest + verb(s) at the end

That is a very standard Dutch sentence pattern.