I weekenden vil vi rydde ud i kælderen og skille os af med ting, vi ikke længere bruger.

This is because Danish is a V2 language in main clauses. That means the finite verb usually comes in the second position.

So when the sentence begins with the time expression I weekenden (at the weekend / this weekend), the verb must come next:

• I weekenden vil vi ...

If you started with the subject instead, you would say:

• Vi vil rydde ud i kælderen i weekenden.

Both are correct, but the version with I weekenden puts more focus on when it will happen.

In this sentence, I weekenden most naturally means this weekend / over the weekend.

Danish often uses the definite form in expressions like this:

• i weekenden = this weekend / at the weekend
• i sommerferien = during the summer holiday
• om vinteren = in winter

If you wanted a more general meaning like on weekends, Danish would more often use something like:

• i weekenderne
• om weekenden in some contexts

So here, I weekenden is best understood as one specific upcoming weekend.

Because Danish often uses the preposition i with time expressions.

Here, i means something like:

• in
• during
• at

So:

• I weekenden = at the weekend / during the weekend

This is just the normal Danish way to form the expression.

Rydde ud means to clear out, to declutter, or to sort through and remove things you do not want anymore.

It is different from just rydde op, which usually means to tidy up.

Compare:

• rydde op = tidy up
• rydde ud = clear out / throw out unwanted things

So in this sentence, rydde ud i kælderen means they are not just making the basement neat—they are going through it and removing things.

Because rydde ud i [a place] is a common Danish expression meaning to clear things out in a place.

• rydde ud i kælderen = clear out the basement
• literally more like clear out in the basement

You can say rydde kælderen, but that sounds more like clear the basement in a direct sense. Rydde ud i kælderen is very natural when talking about sorting through stored things and getting rid of them.

Kælderen is the definite form of kælder.

• en kælder = a basement / cellar
• kælderen = the basement / cellar

Danish usually adds the definite article as an ending:

• bil → bilen
• bog → bogen
• kælder → kælderen

So i kælderen means in the basement.

It is an idiomatic expression meaning to get rid of.

The full expression is:

• skille sig af med noget = get rid of something

In your sentence, the subject is vi, so sig changes to os:

• vi skiller os af med noget = we get rid of something

This expression is best learned as a whole chunk, because its meaning is not obvious from the individual words.

Because the reflexive pronoun changes depending on the subject.

With skille sig af med, the sig part is the dictionary form, but in real sentences it changes:

• jeg skiller mig af med
• du skiller dig af med
• han/hun skiller sig af med
• vi skiller os af med
• I skiller jer af med
• de skiller sig af med

So since the subject here is vi, the correct form is os.

Because that is simply how this fixed expression is built:

• skille sig af med noget

You should think of af med as part of the verbal expression, not as something you can freely change. Danish has many multi-word verbs and idiomatic combinations like this.

So the structure is:

• skille
  • reflexive pronoun + af med
    • object

Example:

• Vi vil skille os af med de gamle møbler.
• She wants to get rid of the old furniture.

Because ting here means things in a general, indefinite sense.

• ting = things
• nogle ting = some things
• de ting = the things

In this sentence, they mean things that we no longer use, not a specific previously mentioned set of things. So plain ting is the natural choice.

Because vi ikke længere bruger is a clause describing ting.

So:

• ting, vi ikke længere bruger
• things that we no longer use

This is a relative clause, even though Danish often leaves out the relative word that English would use, such as that.

You can think of it as:

• ting, som vi ikke længere bruger

The comma helps separate the main noun from the clause that describes it.

It is omitted.

In Danish, relative clauses often leave out som when it would be the object of the clause.

So these are both possible:

• ting, vi ikke længere bruger
• ting, som vi ikke længere bruger

Both mean:

• things that we no longer use

The version without som is very common and natural.

Ikke længere means no longer or not anymore.

So:

• vi ikke længere bruger = we no longer use

It is a very common expression:

• Jeg bor ikke længere i Aarhus. = I no longer live in Aarhus.
• Han arbejder ikke længere her. = He doesn’t work here anymore.

Because this is a subordinate clause, and Danish word order changes in subordinate clauses.

In a main clause:

• Vi bruger ikke længere de ting.

In the subordinate clause after ting:

• ting, vi ikke længere bruger

In subordinate clauses, words like ikke usually come before the finite verb:

• vi ikke længere bruger

That is a very important difference between Danish main-clause and subordinate-clause word order.

Because after a modal verb like vil, Danish uses the infinitive without at.

So:

• vil rydde
• vil skille
• kan komme
• skal gå
• må vente

This is similar to English:

• we will clear out
• not we will to clear out

So vil vi rydde ud ... og skille os af med ... is exactly what you would expect after vil.

Because both infinitives are coordinated under the same modal verb.

So:

• vil vi rydde ud i kælderen og skille os af med ting ...

means:

• we want/will [clear out the basement] and [get rid of things ...]

Danish does not need to repeat vil before the second infinitive unless you want special emphasis.

Yes. Weekend is a borrowed word used in Danish, and the definite form is:

• weekenden = the weekend

So:

• i weekenden = at the weekend / this weekend

This is completely normal modern Danish.