Ich setze das Kind auf das Sofa.

Because setzen and sitzen are different verbs.

• setzen = to set / to seat / to put into a sitting position
• sitzen = to sit / to be sitting

In Ich setze das Kind auf das Sofa, the speaker is doing something to the child, so German uses setzen.

Compare:

• Ich setze das Kind auf das Sofa. = I seat / put the child onto the sofa.
• Das Kind sitzt auf dem Sofa. = The child is sitting on the sofa.

A useful pattern is:

• setzen = movement into a sitting position
• sitzen = the state of already being seated

Kind is a neuter noun in German, so its basic article is das.

• das Kind = the child

In this sentence, das Kind is the direct object of setze, so it is in the accusative case. But for neuter nouns, the article das stays the same in nominative and accusative:

• nominative: das Kind
• accusative: das Kind

So even though the case is accusative here, the form still looks like das Kind.

Because auf is a two-way preposition. That means it can take either:

• accusative for movement toward a destination
• dative for location

Here there is movement: the child is being placed onto the sofa. So German uses the accusative:

• auf das Sofa = onto the sofa

If the child were already there and you were just describing location, you would use dative:

• auf dem Sofa = on the sofa

So the contrast is:

• Ich setze das Kind auf das Sofa. = I put/seat the child onto the sofa.
• Das Kind sitzt auf dem Sofa. = The child is sitting on the sofa.

Because the sentence describes a change of position or motion toward a place.

With two-way prepositions like auf, German asks:

• Where to? → accusative
• Where? → dative

In this sentence, the answer is where to?
The child is being moved onto the sofa, so German uses the accusative:

• auf das Sofa

This is one of the most important patterns to learn with prepositions like auf, in, an, unter, über, vor, hinter, neben, zwischen.

Yes. Auf das is very commonly contracted to aufs.

So these mean the same thing:

• Ich setze das Kind auf das Sofa.
• Ich setze das Kind aufs Sofa.

The contracted form aufs is very natural in everyday German.

The sentence follows the normal German main-clause pattern:

• Ich = subject
• setze = conjugated verb in second position
• das Kind = direct object
• auf das Sofa = prepositional phrase

So the structure is:

Subject + verb + object + place/direction

That gives:

Ich + setze + das Kind + auf das Sofa.

This is a very standard word order in German.

Yes. Jemanden setzen often means to seat someone or to put someone somewhere in a sitting position.

So with a person or child, it is very natural:

• Ich setze das Kind auf das Sofa.
• Sie setzt das Baby in den Kinderstuhl.

For many inanimate objects, German often prefers stellen or legen, depending on position:

• stellen = put something standing/upright
• legen = put something lying down
• setzen = set someone/something into a sitting position

With people, setzen is the normal choice.

You would say:

Das Kind sitzt auf dem Sofa.

This changes two things:

1. setzen becomes sitzen

   • because now you are describing a state, not causing movement

2. auf das Sofa becomes auf dem Sofa

   • because now it is location, not direction

So:

• Ich setze das Kind auf das Sofa. = I put/seat the child onto the sofa.
• Das Kind sitzt auf dem Sofa. = The child is sitting on the sofa.

Yes, but that means something different:

• Ich setze mich auf das Sofa. = I sit down on the sofa / I seat myself on the sofa.

Here mich is reflexive, because the subject and object are the same person.

Compare:

• Ich setze das Kind auf das Sofa. = I put the child onto the sofa.
• Ich setze mich auf das Sofa. = I sit down on the sofa.

So setzen can be used:

• with a direct object: jemanden setzen = seat someone
• reflexively: sich setzen = sit down

The infinitive is setzen.

In the present tense:

• ich setze
• du setzt
• er/sie/es setzt
• wir setzen
• ihr setzt
• sie/Sie setzen

So setze is simply the first-person singular present form:

• Ich setze ... = I set / I am seating / I put

In natural English, often yes.

Even though setzen is closely related to set/seat, English speakers would often say:

• I put the child on the sofa.
• I sat the child on the sofa.
• I seated the child on the sofa.

So the exact English wording depends on context. But grammatically, the German verb is still setzen, because it means causing someone to move into a sitting position.

Because with neuter singular nouns, the article das is the same in both nominative and accusative.

So:

• nominative: das Kind, das Sofa
• accusative: das Kind, das Sofa

That is why the case difference is not visible in the article here.

If the noun were masculine, you would see the change more clearly:

• der Mann → den Mann

For example:

• Ich setze den Mann auf das Sofa.

That makes the accusative easier to spot.