Bevor ich den Kissenbezug in den Schrank lege, falte ich die saubere Bettwäsche zusammen.

Because bevor is a subordinating conjunction. In German, a subordinating conjunction sends the conjugated verb to the end of the clause.

So:

• Bevor ich den Kissenbezug in den Schrank lege
• literally: Before I the pillowcase into the wardrobe put

That is normal German subordinate-clause word order.

A good comparison:

• Ich lege den Kissenbezug in den Schrank.
  = main clause, verb in second position
• Bevor ich den Kissenbezug in den Schrank lege, ...
  = subordinate clause, verb at the end

Because the sentence starts with the bevor-clause, and that whole clause takes up the first position in the sentence. In a German main clause, the conjugated verb must still be in second position, so falte comes next.

Structure:

• Bevor ich den Kissenbezug in den Schrank lege, = position 1
• falte = position 2
• ich = subject after the verb

This is the same pattern as:

• Heute gehe ich nach Hause.
• Wenn es regnet, bleibe ich zu Hause.

So after a fronted clause, German usually has verb + subject.

They are both accusative, but for different reasons.

1. den Kissenbezug

This is the direct object of lege. You are putting the pillowcase somewhere.

• der Kissenbezug = nominative
• den Kissenbezug = accusative

2. in den Schrank

The preposition in can take either dative or accusative.

• accusative = movement/change of location
• dative = location/no movement

Here there is movement into the wardrobe, so German uses accusative:

• in den Schrank = into the wardrobe

Compare:

• Ich lege den Kissenbezug in den Schrank.
  movement → accusative
• Der Kissenbezug liegt im Schrank.
  location → dative (im = in dem)

Because the sentence describes motion into the wardrobe, not a location inside it.

German makes this distinction:

• in den Schrank = into the wardrobe/cupboard
• im Schrank = in the wardrobe/cupboard

So:

• Ich lege den Kissenbezug in den Schrank.
  I put the pillowcase into the wardrobe.
• Der Kissenbezug ist im Schrank.
  The pillowcase is in the wardrobe.

This is a very common pattern with two-way prepositions such as in, auf, unter, neben, etc.

Here zusammenfalten means to fold up or to fold together.

It is a separable verb:

• infinitive: zusammenfalten
• main clause: Ich falte die Bettwäsche zusammen.
• subordinate clause: ..., weil ich die Bettwäsche zusammenfalte.

In a main clause, the prefix zusammen- separates and goes to the end:

• falte ... zusammen

That is why the sentence ends with zusammen.

Not in this exact clause, because this is a main clause with a finite verb.

German separable verbs behave like this:

• Ich falte die Bettwäsche zusammen.
  correct main-clause form
• Ich möchte die Bettwäsche zusammenfalten.
  correct infinitive form
• ..., weil ich die Bettwäsche zusammenfalte.
  correct subordinate-clause form

So in your sentence, falte ... zusammen is the correct form.

Bettwäsche is a feminine singular noun in German, even though in English it may feel like bed linens or bedding, which can sound plural.

So:

• die Bettwäsche = the bedding / bed linen
• saubere Bettwäsche = clean bedding

The adjective ending -e in saubere is there because:

• Bettwäsche is feminine
• it is accusative singular
• it has the definite article die

This gives the weak adjective ending -e:

• die saubere Bettwäsche

In this sentence, it is singular.

That can feel strange to English speakers, because the meaning is often something like bed linen or bedding, which refers to multiple items in real life. But grammatically, Bettwäsche is usually treated as a singular mass/collective noun.

So German says:

• die Bettwäsche ist sauber
  literally: the bedding is clean

If you want to refer to different sets, you might use a plural expression like:

• die Bettwäschen — rare and very context-specific
• more naturally: mehrere Garnituren Bettwäsche = several sets of bed linen

Because it follows the pattern for an adjective after a definite article.

In die saubere Bettwäsche:

• die = definite article
• Bettwäsche = feminine singular
• the whole phrase is accusative singular
• after a definite article, German usually uses weak adjective endings

For feminine accusative singular, the adjective ending is -e:

• die saubere Bettwäsche

Compare:

• die saubere Bettwäsche
• eine saubere Bettwäsche
• saubere Bettwäsche

The endings can change depending on the article, case, gender, and number.

These are examples of compound nouns, and in German the last part of the compound determines the gender.

Kissenbezug

• Kissen = pillow
• Bezug = cover
• the final part is Bezug, which is masculine
• so: der Kissenbezug

Bettwäsche

• Bett = bed
• Wäsche = laundry / linen
• the final part is Wäsche, which is feminine
• so: die Bettwäsche

This is a very useful rule for learning German compound nouns.

This is a very important German pair.

• legen = to lay, to put something somewhere
  → usually takes an object
• liegen = to lie, to be lying
  → describes a position/state

In your sentence:

• Ich lege den Kissenbezug in den Schrank.
  I put the pillowcase into the wardrobe.

But after that action is complete:

• Der Kissenbezug liegt im Schrank.
  The pillowcase is in the wardrobe.

So:

• legen = action/movement
• liegen = position/result

Because the sentence begins with a subordinate clause, and in German subordinate clauses are normally separated from the main clause by a comma.

So:

• Bevor ich den Kissenbezug in den Schrank lege, falte ich die saubere Bettwäsche zusammen.

That comma is required in standard German.

Yes. German allows both orders.

Your sentence:

• Bevor ich den Kissenbezug in den Schrank lege, falte ich die saubere Bettwäsche zusammen.

You can also say:

• Ich falte die saubere Bettwäsche zusammen, bevor ich den Kissenbezug in den Schrank lege.

Both are correct. The difference is mostly about emphasis and style.

• Starting with bevor emphasizes the time relationship right away.
• Starting with Ich falte ... emphasizes the main action first.

It can mean different things depending on context. Schrank is a general word for a large storage piece of furniture.

Possible translations include:

• wardrobe
• cabinet
• cupboard
• sometimes even closet in looser translation

In this sentence, wardrobe or cupboard/cabinet could all work depending on what kind of furniture you imagine. The German word itself is broader than any single English word.

Because bevor introduces a clause.

• bevor = before, when followed by a full clause
  Bevor ich den Kissenbezug in den Schrank lege, ...

By contrast:

• vor is a preposition, so it must be followed by a noun phrase, not a clause
  example: vor dem Essen = before the meal
• vorher is an adverb meaning beforehand / earlier
  example: Ich falte die Bettwäsche vorher zusammen.

So if you want to say before I put the pillowcase in the wardrobe, you need bevor.