Wenn der Stuhl wackelt, setze ich mich lieber auf das Sofa.

Wenn introduces a subordinate clause and here means if or when, depending on context.

In this sentence, Wenn der Stuhl wackelt can be understood as:

• If the chair wobbles...
• When the chair wobbles...

German wenn often covers both ideas when talking about a condition or a repeated situation. If you wanted a more clearly one-time if, German can also use falls, but wenn is very common.

Because wenn creates a subordinate clause, and in German subordinate clauses the conjugated verb normally goes to the end.

So:

• Main clause: Der Stuhl wackelt.
• Subordinate clause: wenn der Stuhl wackelt

That final-verb position is one of the most important word-order patterns in German.

Because the sentence begins with the wenn clause:

• Wenn der Stuhl wackelt, ...

When a subordinate clause comes first, it takes the first position in the sentence as a whole. In the following main clause, the conjugated verb must then come immediately after it.

So German word order becomes:

• Wenn der Stuhl wackelt, setze ich mich lieber auf das Sofa.

This is the normal verb-second rule in main clauses. The whole wenn clause counts as position 1, so setze comes in position 2.

Because in German, a subordinate clause is separated from the main clause by a comma.

So:

• Wenn der Stuhl wackelt, setze ich mich lieber auf das Sofa.

This comma is required in standard German.

This is a very common question.

• sitzen = to be sitting
• sich setzen = to sit down

So:

• Ich sitze auf dem Sofa. = I am sitting on the sofa.
• Ich setze mich auf das Sofa. = I sit down onto the sofa.

In your sentence, the speaker is choosing where to sit down, so German uses sich setzen, not sitzen.

Because the verb here is sich setzen, which is a reflexive verb.

The full infinitive is:

• sich setzen = to sit down

So with ich, it becomes:

• ich setze mich

The reflexive pronoun changes depending on the subject:

• ich setze mich
• du setzt dich
• er/sie/es setzt sich
• wir setzen uns

English usually does not say I sit myself down in normal speech, but German often uses this reflexive structure.

Because sich setzen takes a reflexive pronoun in the accusative case.

So:

• ich setze mich
• du setzt dich
• er setzt sich

Some reflexive verbs use dative pronouns, but sich setzen uses accusative.

A useful contrast:

• Ich wasche mich. = I wash myself.
• Ich wasche mir die Hände. = I wash my hands.

In the second example, mir is dative because die Hände is the accusative object. In sich setzen, there is no separate accusative object, so the reflexive pronoun itself is accusative: mich.

Lieber means rather or preferably.

So ich setze mich lieber auf das Sofa means the speaker prefers the sofa over the chair.

It comes from gern:

• gern = gladly / like to
• lieber = rather / prefer
• am liebsten = most of all

Examples:

• Ich trinke gern Tee. = I like drinking tea.
• Ich trinke lieber Tee als Kaffee. = I prefer tea to coffee.
• Am liebsten trinke ich Tee. = Tea is what I like best.

Because auf is a two-way preposition, and here the sentence expresses movement toward a destination.

German uses:

• accusative for movement/change of position
• dative for location/static position

So:

• Ich setze mich auf das Sofa. = I sit down onto the sofa.
  
  • movement, so accusative
• Ich sitze auf dem Sofa. = I am sitting on the sofa.
  
  • location, so dative

That is why das Sofa becomes auf das Sofa here.

Yes. Auf das is very often contracted to aufs in everyday German.

So both are possible:

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

Both are correct. The full form auf das Sofa may sound a bit more explicit or careful, while aufs Sofa is very natural in speech.

Yes. The articles show both gender and case.

• der Stuhl

  • Stuhl is masculine
  • here it is the subject of wackelt
  • so it is in the nominative: der Stuhl

• das Sofa

  • Sofa is neuter
  • it comes after auf with movement
  • so it is in the accusative
  • for neuter, nominative and accusative are both das, so the form does not change

That is why you see:

• der Stuhl
• das Sofa

Yes. You can reverse the order:

• Ich setze mich lieber auf das Sofa, wenn der Stuhl wackelt.

This means the same thing. The difference is mainly emphasis and sentence flow.

Compare:

• Wenn der Stuhl wackelt, setze ich mich lieber auf das Sofa.
  
  • starts with the condition
• Ich setze mich lieber auf das Sofa, wenn der Stuhl wackelt.
  
  • starts with the action

Both are natural.

Yes. wackeln is a very natural verb for something that is shaky, loose, or unsteady.

For example:

• Der Tisch wackelt. = The table wobbles.
• Der Zahn wackelt. = The tooth is loose.
• Der Stuhl wackelt. = The chair wobbles.

It suggests that the chair is unstable, not firmly standing.

German often does not need an explicit word like then after a wenn clause. The relationship is understood automatically.

English often says:

• If the chair wobbles, then I’d rather sit on the sofa.

German usually just says:

• Wenn der Stuhl wackelt, setze ich mich lieber auf das Sofa.

You could add words for emphasis in some contexts, but normally they are unnecessary.