De kinderen zitten rustig in de klas.

Because kinderen is plural. In Dutch, all plural nouns take the article de, even if the singular noun is a het-word.

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

So even though kind is singular het, its plural is always de.

Kinderen is an irregular plural. You cannot just add -s or -en to get kinderen by a simple rule; this is just the correct plural form you need to learn.

• het kind = the child
• de kinderen = the children

Dutch has a number of common irregular plurals, and kind → kinderen is one of the most important ones.

Yes, zitten literally means to sit, but in Dutch it is often used more naturally than in English in situations where people are seated somewhere.

So De kinderen zitten rustig in de klas literally means:

• The children are sitting quietly in the classroom

In context, it can also sound like:

• The children are sitting quietly in class
• The children are quietly in the classroom, if the emphasis is on their seated presence there

Dutch often uses verbs like zitten, staan, and liggen where English might simply use be.

Because Dutch often prefers a position verb when it is relevant how someone is located.

Here, the children are understood to be seated, so zitten is natural.

Compare:

• De kinderen zijn in de klas. = The children are in the classroom.
  
  • neutral: just location
• De kinderen zitten in de klas. = The children are sitting in the classroom.
  
  • more specific: they are seated there

So zitten gives a more concrete picture than zijn.

Because rustig is being used here as an adverb, not an adjective.

It describes how they are sitting:

• zitten rustig = sit quietly

Compare:

• de rustige kinderen = the quiet children
  
  • here rustige is an adjective describing kinderen
• de kinderen zitten rustig = the children sit quietly
  
  • here rustig is an adverb describing zitten

In Dutch, adverbs usually do not take the -e ending.

Yes, Dutch adverbs can sometimes move around, but De kinderen zitten rustig in de klas is a very natural order.

This sentence is built like this:

• De kinderen = subject
• zitten = finite verb
• rustig = adverb
• in de klas = prepositional phrase of place

You may also hear:

• De kinderen zitten in de klas rustig.
  
  • grammatically possible in some contexts, but less natural here
• Rustig zitten de kinderen in de klas.
  
  • unusual, marked, and only used for special emphasis

So for a learner, subject + verb + adverb + place is a good pattern to remember in a basic sentence like this.

It can mean either, depending on context.

Literally:

• in de klas = in the classroom

But in natural English, it may sometimes be translated as:

• in class

Dutch often uses de klas both for the physical room and for the classroom setting more generally.

So the most literal translation is in the classroom, but in class may be the more natural English translation in some situations.

Because klas is a de-word.

• de klas = the class / the classroom

Unlike English, Dutch nouns have grammatical gender/categories, and you usually have to learn whether a noun takes de or het.

So:

• de klas
• in de klas = in the classroom

Because Dutch main clauses usually follow the V2 rule: the finite verb comes in the second position.

In this sentence:

• De kinderen = first element
• zitten = finite verb in second position
• rustig in de klas = the rest of the sentence

So the structure is:

• De kinderen | zitten | rustig in de klas.

This is one of the most important basic word-order rules in Dutch.

Yes. In a subordinate clause, the finite verb usually moves toward the end.

Main clause:

• De kinderen zitten rustig in de klas.

Subordinate clause:

• Ik zie dat de kinderen rustig in de klas zitten.
  = I see that the children are sitting quietly in the classroom.

Notice that zitten comes at the end of the subordinate clause after dat.

A helpful approximate pronunciation is:

• KIN-duh-run

More carefully in standard Dutch:

• /ˈkɪn.də.rə(n)/

A few useful points:

• the stress is on the first syllable: KIN-
• the d is pronounced
• the e sounds are reduced, like a soft uh
• the final -n may be weak or sometimes barely heard in casual speech

Yes. Kinderen is a general word meaning children, regardless of gender.

If you want to be more specific, you could say:

• jongens = boys
• meisjes = girls

But kinderen naturally includes both.

Yes. Zitten here is the present tense plural form of zitten.

The singular forms are:

• ik zit = I sit / I am sitting
• jij zit = you sit / you are sitting
• hij/zij/het zit = he/she/it sits / is sitting

Plural:

• wij zitten
• jullie zitten
• zij zitten

Since de kinderen is plural, the verb is zitten.

Yes, but it means something slightly different.

• De kinderen zitten rustig in de klas.

  • The children are sitting quietly in the classroom.
  • This focuses on their seated position and behavior.

• De kinderen zijn rustig in de klas.

  • The children are calm/quiet in the classroom.
  • This focuses more on their state or behavior in general.

So both are possible, but zitten paints a more specific physical scene.