Tom loopt stil door het huis, want Anna slaapt.

Because Tom is third person singular (he), and in the present tense Dutch usually adds -t to the verb for jij/u/hij/zij/het.

So with the verb lopen (to walk), you get:

• ik loop
• jij loopt
• hij loopt
• Tom loopt
• wij lopen

So Tom loopt means Tom walks / is walking.

It can mean both, depending on context.

Dutch often uses the simple present where English might use either:

• Tom walks quietly through the house
• Tom is walking quietly through the house

In this sentence, because it describes what is happening right now, English will often translate it as is walking.

In Dutch, stil often means quiet or silent, not just still in the English sense.

In this sentence, stil works like quietly:

• Tom loopt stil door het huis = Tom walks quietly through the house

So this is a very common Dutch pattern: an adjective can sometimes be used where English prefers an adverb.

Because stil is not describing a noun here. It is describing how Tom walks.

Compare:

• de stille straat = the quiet street
  Here stille describes the noun straat, so it gets an ending.

• Tom loopt stil = Tom walks quietly
  Here stil describes the action, so it stays in its basic form.

This is very common in Dutch: predicative and adverb-like uses usually do not take -e.

Door het huis means through the house.

• door = through
• het huis = the house

It suggests movement from one part of the house to another, not just being inside it.

So:

• in het huis = in the house (location)
• door het huis = through the house (movement)

Because huis is a het-word in Dutch.

Dutch nouns take one of two definite articles:

• de
• het

You simply have to learn which article belongs to each noun:

• de straat
• de man
• het huis
• het boek

So the correct form is het huis.

Because want introduces a new clause meaning because / for, and in Dutch it is normally preceded by a comma.

So the sentence is divided into:

• Tom loopt stil door het huis
• want Anna slaapt

The comma helps show the second clause gives the reason.

Both can mean because, but they behave differently in word order.

With want, the following clause keeps main clause word order:

• Tom loopt stil door het huis, want Anna slaapt.

With omdat, the verb usually goes to the end of the clause:

• Tom loopt stil door het huis, omdat Anna slaapt.

So:

• want Anna slaapt
• omdat Anna slaapt

In this particular sentence, both are possible, but want is often a bit more conversational and straightforward.

Because want does not create subordinate clause word order.

After want, Dutch keeps normal main-clause order:

• want Anna slaapt

That is different from conjunctions like omdat, where the verb goes to the end:

• omdat Anna slaapt

So if you see want, expect a more normal subject-verb pattern.

Dutch main clauses usually follow the verb-second (V2) rule. That means the conjugated verb comes in the second position.

Here:

1. Tom
2. loopt

Then the rest follows:

• stil
• door het huis

So the structure is basically:

• Subject + finite verb + rest

If you moved another element to the front, the verb would still stay second:

• Stil loopt Tom door het huis.
  This is grammatical, but less natural in ordinary speech.

Yes. That would also be natural.

There is a slight nuance:

• stil = quietly, silently, with little or no noise
• zachtjes = softly, gently, quietly

In this sentence, stil fits very well because Anna is sleeping, so Tom is trying not to make noise.

Yes.

The infinitive is slapen = to sleep.
For Anna (she, third person singular), the present tense is slaapt.

Conjugation:

• ik slaap
• jij slaapt
• hij/zij slaapt
• Anna slaapt
• wij slapen

So Anna slaapt means Anna sleeps or, more naturally here, Anna is sleeping.

A natural English version would be:

• Tom is walking quietly through the house because Anna is sleeping.

Or, keeping the Dutch structure more closely:

• Tom walks quietly through the house, because Anna is sleeping.

Dutch uses the simple present more broadly than English, so is walking and is sleeping often sound more natural in English for this kind of situation.