Jeg sætter moppen tilbage i skabet, når gulvet er rent.

Sætte is the infinitive, meaning to put / to place / to set.

In this sentence, the subject is jeg = I, and the verb is being used as a present-tense finite verb, so it becomes sætter:

• at sætte = to put
• jeg sætter = I put / I am putting

Danish verbs do not change for person the way English verbs sometimes do. So the same present-tense form is used with all subjects:

• jeg sætter
• du sætter
• han/hun sætter
• vi sætter

Because Danish usually adds the definite article to the end of the noun instead of putting a separate word like the in front.

So:

• en moppe = a mop

• moppen = the mop

• et gulv = a floor

• gulvet = the floor

• et skab = a cupboard / cabinet
• skabet = the cupboard / cabinet

The ending depends on the noun’s gender:

• common gender nouns usually take -en
• neuter nouns usually take -et

This is one of the biggest differences from English.

Because rent agrees with gulvet, and gulv is a neuter noun (et gulv).

The adjective ren = clean changes form depending on what it describes:

• en ren moppe = a clean mop
• et rent gulv = a clean floor
• rene gulve = clean floors

After er, the adjective still agrees with the noun:

• gulvet er rent = the floor is clean

So rent is used because gulvet is singular neuter.

Tilbage means back.

So sætter ... tilbage means something like:

• put back
• return ... to its place

In this sentence, Jeg sætter moppen tilbage i skabet means that the mop is being put back where it belongs, not just placed somewhere randomly.

This is a very common question.

In Danish, i can often cover both:

• in
• into

The movement meaning is often understood from the verb and the context.

So:

• Moppen er i skabet = The mop is in the cupboard
• Jeg sætter moppen i skabet = I put the mop in/into the cupboard

Because sætter already implies movement, Danish does not always need a separate word for into. If you want to make the movement more explicit, Danish can also say ind i skabet, but i skabet is very normal.

Når usually means when, while hvis usually means if.

In this sentence, når gulvet er rent means when the floor is clean. That suggests a time situation, not a condition being tested.

Compare:

• Jeg sætter moppen tilbage i skabet, når gulvet er rent.
  = I put the mop back in the cupboard when the floor is clean.

• Jeg sætter moppen tilbage i skabet, hvis gulvet er rent.
  = I put the mop back in the cupboard if the floor is clean.

The version with hvis sounds more conditional. The version with når sounds like a normal sequence or routine.

Because Danish normally uses a comma before a subordinate clause.

Here, når gulvet er rent is a subordinate clause introduced by når.

So the sentence is split into:

• main clause: Jeg sætter moppen tilbage i skabet
• subordinate clause: når gulvet er rent

Danish comma rules are generally stricter than English ones in this kind of structure, so that comma is expected.

Because subordinate clauses in Danish usually do not have the same inversion patterns that main clauses can have.

In the subordinate clause:

• gulvet = subject
• er = verb
• rent = adjective

So the normal order is:

• når gulvet er rent

If this were a question, then you could get:

• Er gulvet rent? = Is the floor clean?

But after når, it is a statement clause, so Danish keeps the regular subject + verb order.

It can be understood in more than one way, and that is normal in Danish.

The present tense in Danish often covers:

• something happening now
• a general habit
• a future action, especially in everyday speech

So Jeg sætter moppen tilbage i skabet, når gulvet er rent can mean:

• I put the mop back in the cupboard when the floor is clean
• I’ll put the mop back in the cupboard when the floor is clean
• I usually put the mop back in the cupboard when the floor is clean

Context tells you which one is meant.

Yes, absolutely. That is a very natural alternative.

Both are correct:

• Jeg sætter moppen tilbage i skabet, når gulvet er rent.
• Når gulvet er rent, sætter jeg moppen tilbage i skabet.

The difference is mainly focus:

• starting with Jeg sætter... puts the main action first
• starting with Når gulvet er rent... emphasizes the time condition first

Notice something important in the second version: after the fronted subordinate clause, the verb comes before the subject in the main clause:

• Når gulvet er rent, sætter jeg ...

That is standard Danish V2 word order in main clauses.

Skab is a broad word. It can refer to different kinds of enclosed storage furniture, depending on context.

So i skabet could mean:

• in the cupboard
• in the cabinet
• in the closet
• in the storage unit

In a cleaning context, many learners will understand it as the cupboard or the cleaning cabinet. The exact English word depends on what kind of skab the speaker has in mind.