Wenn der Trockner fertig ist, räume ich den Wäscheständer weg.

Because wenn introduces a subordinate clause. In German, the conjugated verb usually goes to the end of a subordinate clause.

So the pattern is:

• Wenn
  • subject + other elements + verb
• Wenn der Trockner fertig ist

That is one of the biggest word-order differences from English.

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

Here, the entire wenn-clause comes first:

• Wenn der Trockner fertig ist, ...

That whole clause counts as position 1. So the verb of the main clause must come next:

• räume = position 2
• ich comes after it

So:

• Wenn der Trockner fertig ist, räume ich den Wäscheständer weg.

If you start with the main clause instead, you get normal subject-verb order:

• Ich räume den Wäscheständer weg, wenn der Trockner fertig ist.

Here, wenn means when in the sense of a future or repeated situation.

German uses:

• wenn for when/if in subordinate clauses, especially for future, general, or repeated situations
• wann for when? in a question or indirect question
• als for when referring to a single event in the past

So:

• Wenn der Trockner fertig ist = when the dryer is done
• Wann ist der Trockner fertig? = when is the dryer done?
• Als der Trockner fertig war = when the dryer was done / when the dryer finished, in a past one-time situation

Because they are in different cases.

• der Trockner is the subject of the subordinate clause, so it is in the nominative
• den Wäscheständer is the direct object of wegräumen, so it is in the accusative

Since Wäscheständer is masculine, the article changes:

• nominative: der Wäscheständer
• accusative: den Wäscheständer

This sentence is a good example of how German articles show grammatical role.

It belongs to the verb wegräumen, which is a separable verb.

The full verb is:

• wegräumen = to put away / clear away

In a normal main clause, the prefix separates:

• Ich räume den Wäscheständer weg.

So:

• räume = conjugated verb part
• weg = separable prefix, moved to the end

That is completely normal for separable verbs in main clauses.

Because in a main clause, a separable verb splits apart.

Compare:

• Main clause: Ich räume den Wäscheständer weg.
• Subordinate clause: ..., weil ich den Wäscheständer wegräume.

So:

• in a main clause: räumen ... weg
• in a subordinate clause with the verb at the end: wegräume as one word

That is a very common pattern in German.

Yes, fertig is an adjective here, but it is being used predicatively with sein.

Predicative adjectives do not take adjective endings:

• Der Trockner ist fertig.

Compare that with an adjective before a noun, where endings do appear:

• die fertige Wäsche

So in your sentence, fertig has no ending because it comes after sein.

Because German often uses the present tense to talk about the future when the context already makes the timing clear.

In this sentence, the future meaning is obvious from the situation:

• Wenn der Trockner fertig ist, räume ich den Wäscheständer weg.

English often does something similar:

• When the dryer is done, I put the drying rack away.

German does not need a future form here. You could use werden in some contexts, but it would usually sound less natural in this everyday sentence.

Yes. In German, a subordinate clause introduced by wenn must be separated by a comma.

So this is correct:

• Wenn der Trockner fertig ist, räume ich den Wäscheständer weg.

And if the order is reversed, the comma is still needed:

• Ich räume den Wäscheständer weg, wenn der Trockner fertig ist.

German comma rules are stricter than English ones in this area.

Yes:

• Ich räume den Wäscheständer weg, wenn der Trockner fertig ist.

That means the same thing.

The main difference is focus or flow:

• starting with Wenn der Trockner fertig ist emphasizes the time/condition first
• starting with Ich räume den Wäscheständer weg emphasizes the action first

The grammar changes slightly in the main clause:

• after a fronted subordinate clause: räume ich
• with the subject first: ich räume

German often builds nouns by combining smaller words.

• Wäscheständer = Wäsche
  • Ständer
• Trockner comes from the verb trocknen and means something like a dryer/device for drying

A useful rule: the last part of a German compound usually determines the gender.

So:

• der Ständer → der Wäscheständer
• der Trockner is also masculine

That is why both nouns use masculine articles in this sentence.

It means something like put away, clear away, or move out of the way.

The prefix weg- adds the idea of away. So räumen by itself is related to clearing or tidying, and wegräumen specifically suggests removing something because it is no longer needed where it is.

In this sentence, that fits well: once the dryer is done, the drying rack gets put away.

Yes, often you could say:

• Sobald der Trockner fertig ist, räume ich den Wäscheständer weg.

Sobald means as soon as, so it sounds a bit more immediate.

• wenn = when
• sobald = as soon as

Your original sentence with wenn is completely natural. Sobald just makes the timing feel a little tighter.