Sobald der Kissenbezug trocken ist, lege ich ihn auf das Bett und gehe früher schlafen.

Because sobald introduces a subordinate clause. In German, subordinate clauses normally send the conjugated verb to the end.

So:

• der Kissenbezug ist trocken = main clause
• sobald der Kissenbezug trocken ist = subordinate clause

That is why you get trocken ist, not ist trocken.

German main clauses follow the verb-second rule: the conjugated verb must be in the second position.

Here, the whole subordinate clause comes first:

• Sobald der Kissenbezug trocken ist, ...

That entire clause takes up position 1. So the verb of the main clause must come next:

• lege ich ihn auf das Bett

This is very common in German:

• Wenn ich Zeit habe, gehe ich spazieren.
• Nach der Arbeit esse ich etwas.

So lege ich is correct because the first slot is already occupied.

Der Kissenbezug is the subject of the subordinate clause, so it is in the nominative case.

Also, Kissenbezug is a masculine noun, so its nominative singular article is der.

A useful detail: in German compound nouns, the last part usually determines the gender. Here the last part is Bezug, which is masculine, so:

• der Bezug
• der Kissenbezug

Ihn refers back to der Kissenbezug.

Since Kissenbezug is:

• masculine
• singular
• the direct object of lege

the pronoun must be accusative masculine singular:

• der → ihn

So:

• der Kissenbezug = nominative
• ich lege ihn = accusative object

Because trocken is being used as a predicate adjective after sein.

In German, adjectives used after verbs like sein, werden, and bleiben do not take adjective endings.

So:

• Der Kissenbezug ist trocken.
• Das Bett ist weich.
• Die Schuhe sind sauber.

But if the adjective comes before a noun, then it does take an ending:

• der trockene Kissenbezug

So trocken ist is correct.

Because German normally uses a comma to separate a subordinate clause from a main clause.

So here:

• Sobald der Kissenbezug trocken ist, = subordinate clause
• lege ich ihn auf das Bett und gehe früher schlafen. = main clause

This comma is required in standard German.

Because legen expresses movement to a destination, not location.

German often distinguishes:

• accusative for direction / movement toward
• dative for location

So:

• Ich lege ihn auf das Bett. = I put it onto the bed. → accusative
• Er liegt auf dem Bett. = It is lying on the bed. → dative

That is why auf das Bett is used here.

Not with the same meaning.

• auf das Bett = onto the bed, on top of it
• ins Bett = into bed

So if you are putting the pillowcase on the bed, then auf das Bett is right.

If you said ich gehe früher ins Bett, that would mean I go to bed earlier, which is a different action.

Because schlafen gehen is a common German verb combination meaning to go to sleep / go to bed. In this structure, German does not use zu.

So:

• Ich gehe schlafen. = I’m going to sleep.
• Ich gehe früher schlafen. = I’m going to sleep earlier.

This is similar to other motion-verb combinations where the second verb appears as a bare infinitive.

Früher here means earlier in time, and it is the natural choice for talking about going to sleep at an earlier time.

• Ich gehe früher schlafen. = I go to sleep earlier.

Eher can sometimes mean rather / sooner / more likely, but in this sentence früher is the most straightforward word.

Yes. German very often uses the present tense to talk about the future when the meaning is clear from context.

Here the time reference is already clear because of sobald:

• Sobald der Kissenbezug trocken ist ... = as soon as the pillowcase is dry ...

So German does not need a special future form here. This is very normal.

Both can refer to a future situation, but sobald means as soon as, so it sounds more immediate and specific.

• Sobald der Kissenbezug trocken ist ... = As soon as the pillowcase is dry ...
• Wenn der Kissenbezug trocken ist ... = When the pillowcase is dry ...

So sobald emphasizes that the second action happens immediately after the first condition is met.

Because once ich has already been stated in the main clause, German can leave it out in the second coordinated verb phrase if the subject is the same.

So:

• lege ich ihn auf das Bett und gehe früher schlafen

means:

• I put it on the bed and go to sleep earlier

You could think of the second part as sharing the same subject:

• [ich] lege ... und [ich] gehe ...

This is normal and natural German.