Ik hoorde Sofie zacht mompelen dat ze moest blozen toen iedereen naar haar keek.

After verbs of perception such as horen (to hear), zien (to see), and voelen (to feel), Dutch normally uses a bare infinitive: no te.

So:

• Ik hoorde Sofie mompelen = I heard Sofie muttering
• Ik zag hem weglopen = I saw him walk away / walking away

This is similar to English, which also often uses a bare verb after perception verbs:

• I heard Sofie mutter
• I saw him leave

So hoorde ... mompelen is the normal structure here.

It is really doing both jobs at once.

In Ik hoorde Sofie zacht mompelen:

• Ik is the subject of hoorde
• Sofie is the person I heard
• mompelen is what Sofie was doing

So Sofie is the object of hoorde, but at the same time the logical subject of mompelen.

You can think of it like this:

• I heard [Sofie] [mutter softly]

This is a very common pattern after horen, zien, etc.

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

It describes how Sofie was muttering:

• zacht mompelen = to mutter softly

Dutch often uses the same form for adjectives and adverbs, unlike English.

Compare:

• een zachte stem = a soft voice
  → adjective
• ze sprak zacht = she spoke softly
  → adverb

So zacht here is completely normal.

Yes, zachtjes would also be possible:

• Ik hoorde Sofie zachtjes mompelen

Both zacht and zachtjes can mean softly/quietly here.

Very roughly:

• zacht is a bit more neutral
• zachtjes can sound a little more informal, gentle, or vivid

In this sentence, both work well.

Here moest is the past tense of moeten.

But moeten does not always mean strict obligation like English must. In a sentence like this, it can express something like:

• she had to blush
• she couldn’t help blushing
• she ended up blushing

So dat ze moest blozen suggests that because of the situation, blushing was almost unavoidable.

It does not necessarily mean someone told her she was required to blush. It is more about an inevitable reaction.

Because blozen depends on moest.

In Dutch, modal verbs such as moeten, kunnen, willen, mogen, and zullen are usually followed by an infinitive:

• ze moest blozen
• ik kon niet slapen
• wij wilden gaan

So:

• moest = finite verb
• blozen = infinitive

That is why you do not get ze moest bloosde.

Both ze and zij can mean she (or they, depending on context).

Here ze is the unstressed form, which is the normal choice in the middle of a sentence:

• dat ze moest blozen

You would use zij more often when the pronoun is stressed, contrasted, or emphasized:

• Niet Anna, maar zij moest blozen.
• Zij moest blozen, niet ik.

So in this sentence, ze is the natural form.

In normal reading, yes.

The sentence says:

• Ik hoorde Sofie zacht mompelen dat ze moest blozen...

The most natural interpretation is that ze refers back to Sofie.

Dutch, like English, often uses a pronoun when the reference is clear from context. Since Sofie is the most obvious female person mentioned, ze is understood as Sofie.

Because both parts are subordinate clauses.

In Dutch subordinate clauses introduced by words like:

• dat = that
• toen = when
• omdat = because
• als = if/when

the verb typically moves toward the end.

So:

Main clause

• Ik hoorde Sofie zacht mompelen ...

Subordinate clause 1

• dat ze moest blozen
• literally: that she had-to blush

Subordinate clause 2

• toen iedereen naar haar keek
• literally: when everyone at her looked

This verb-final tendency is one of the most important word-order features in Dutch.

Because the verb here is kijken naar = to look at.

So Dutch uses a preposition:

• naar haar kijken = to look at her

That is why you get:

• toen iedereen naar haar keek

You cannot normally say haar keek with kijken.

Compare:

• ik kijk naar de lucht = I look at the sky
• ze keken naar hem = they looked at him

Yes, aankijken is also possible:

• toen iedereen haar aankeek

Both are possible, but there is a slight difference in feel.

• naar haar kijken = to look at her
• haar aankijken = to look at her directly / to direct one’s gaze at her

In many contexts they are close in meaning, but aankijken can sound a bit more direct or focused.

The version with naar haar keek is perfectly natural.

Because iedereen is grammatically singular in Dutch, even though it refers to many people.

So you say:

• iedereen is
• iedereen heeft
• iedereen kijkt
• iedereen keek

This is similar to English everyone is, not everyone are.

So:

• toen iedereen naar haar keek = when everyone was looking at her

is grammatically singular.

Toen is used for a specific event in the past.

That fits this sentence, because it describes one particular moment:

• toen iedereen naar haar keek = when everyone looked / was looking at her

Dutch usually uses:

• toen for specific past events
• als for repeated events, general truths, or conditions

Compare:

• Toen ik hem zag, zwaaide ik. = When I saw him, I waved.
  (one specific past moment)

• Als ik hem zie, zwaai ik. = When I see him, I wave.
  (general/repeated)

So toen is the right choice here.

Both are possible, but they are structured differently.

1. The sentence you have:

• Ik hoorde Sofie zacht mompelen dat ...

This focuses on hearing Sofie in the act of muttering.

2. Another possible sentence:

• Ik hoorde dat Sofie zacht mompelde ...

This means more literally I heard that Sofie was muttering..., which can sound less direct and a bit more like you received that information somehow.

So the original version is more vivid and more natural if you literally heard her speaking.

In Dutch, commas are used somewhat differently from English, and a comma before a subordinate clause is often not required.

So this is normal:

• Ik hoorde Sofie zacht mompelen dat ze moest blozen...

A comma might sometimes appear for clarity or style in longer sentences, but here leaving it out is standard and natural.

English often uses commas more readily in long sentences, but Dutch is usually more restrained.

Hoorde is the simple past (also called the imperfect) of horen.

• ik hoor = I hear
• ik hoorde = I heard / I was hearing

Dutch often uses the simple past for narration, especially in written language or storytelling.

So Ik hoorde Sofie... is a normal narrative past form.

A rough literal breakdown is:

• Ik = I
• hoorde = heard
• Sofie = Sofie
• zacht = softly
• mompelen = mutter
• dat = that
• ze = she
• moest = had to
• blozen = blush
• toen = when
• iedereen = everyone
• naar haar = at her
• keek = looked

So a very literal version would be:

I heard Sofie softly mutter that she had to blush when everyone looked at her.

That helps show the Dutch structure, especially the verb placement at the end of the subordinate clauses.