Die Kleider trocknen schnell in der Sonne.

Because Kleider is a plural noun, and die is the definite article for the nominative plural.

Here, Die Kleider is the subject of the sentence, so nominative is needed.

A quick comparison:

• das Kleid = the dress / the garment
• die Kleider = the dresses / the clothes

So die here does not mean feminine singular; it means plural.

It can be, but this is a point learners often notice.

• das Kleid usually means dress
• die Kleider literally means dresses
• but in some contexts, die Kleider can also mean clothes / garments

In everyday German, people often also use:

• die Kleidung = clothing
• die Klamotten = clothes, more informal

So this sentence is grammatically fine, but depending on context, a native speaker might also say:

• Die Kleidung trocknet schnell in der Sonne.
• Die Klamotten trocknen schnell in der Sonne.

Because trocknen can be used intransitively here, meaning to dry in the sense of to become dry.

So:

• Die Kleider trocknen. = The clothes are drying / The clothes dry.

German does not need a separate verb like are drying in this basic sentence. The simple present can express that idea.

Compare:

• Ich trockne die Kleider. = I dry the clothes.
  Here it is transitive: someone dries something.

• Die Kleider trocknen. = The clothes dry / are drying.
  Here it is intransitive: the clothes themselves become dry.

Because the subject is plural: die Kleider.

The verb trocknen is conjugated like this in the present tense:

• ich trockne
• du trocknest
• er/sie/es trocknet
• wir trocknen
• ihr trocknet
• sie/Sie trocknen

Since die Kleider is a third-person plural subject, the correct form is trocknen.

Because schnell is being used as an adverb, not an adjective.

It describes how the drying happens:

• Die Kleider trocknen schnell. = The clothes dry quickly.

Adverbs in German do not take adjective endings.

Compare:

• die schnellen Kleider = the fast clothes
  Here schnellen is an adjective before a noun, so it gets an ending.

• Die Kleider trocknen schnell.
  Here schnell modifies the verb trocknen, so it stays unchanged.

Because this phrase expresses location, not movement toward a destination.

With in, German uses:

• dative for where?
• accusative for where to?

Here the clothes are drying in the sun, meaning in the sunlight / in that location, so German uses the dative:

• in der Sonne

Compare:

• Die Kleider trocknen in der Sonne. = location
• Ich lege die Kleider in die Sonne. = movement into the sun/sunlight

So der is the dative singular form of die Sonne.

Because German idiomatically says in der Sonne to mean in the sun / in sunlight.

This works very much like English in the sun. It does not mean the clothes are physically inside the sun; it means they are exposed to sunshine.

So:

• in der Sonne = in the sun
• in the sunlight / where the sun shines

It is the normal expression here.

German main clauses put the conjugated verb in the second position, so trocknen must come early:

• Die Kleider | trocknen | schnell | in der Sonne

After the verb, German often places adverbs in a fairly flexible order, but schnell before in der Sonne sounds very natural.

Here:

• schnell = how? how fast?
• in der Sonne = where?

So the sentence flows naturally as:

• subject
• verb
• manner
• place

You could also hear:

• Die Kleider trocknen in der Sonne schnell.

But that is less natural in neutral everyday German. schnell before in der Sonne is the better default.

Yes. Because Kleider is the plural of Kleid, it can definitely mean dresses.

So without context, Die Kleider could refer to:

• several dresses
• clothes/garments more generally

Context tells you which meaning is intended.

In German, all nouns are capitalized.

So in this sentence:

• Kleider is capitalized because it is a noun.
• Sonne is capitalized because it is also a noun.

But words like:

• die
• trocknen
• schnell
• in
• der

are not nouns, so they are not capitalized.

This is a standard rule in German spelling.

Yes. German present tense often covers both meanings.

So Die Kleider trocknen schnell in der Sonne can mean:

• a general fact: clothes dry quickly in the sun
• a current situation: the clothes are drying quickly in the sun

German often does not need a separate form like English are drying. The present tense does both jobs, and context makes the meaning clear.