Welches Waschmittel benutzt du für dunkle Kleidung?

Because Waschmittel is a neuter singular noun: das Waschmittel.

The question word welch- changes its ending to match the noun it goes with. Here it matches a neuter singular noun, so you get welches Waschmittel.

Also, Waschmittel is the direct object of benutzen, so it is in the accusative case. In the neuter singular, nominative and accusative both look the same here: welches.

Yes. Was für ein Waschmittel benutzt du für dunkle Kleidung? is also natural.

The difference is mainly one of style:

• welches Waschmittel = which detergent
• was für ein Waschmittel = what kind of detergent

In everyday speech, was für ein is often a bit more conversational. Welches can sound slightly more like you are choosing from known options.

It is in the accusative case, because it is the thing being used.

• du benutzt etwas
• that etwas is the direct object
• direct objects after benutzen take the accusative

So in this sentence:

• du = subject
• benutzt = verb
• welches Waschmittel = direct object in the accusative

Because this is a wh-question in German.

German main-clause word order usually puts the finite verb in second position. In a question with a question word, the question phrase comes first, and the verb comes right after it.

So the order is:

• Welches Waschmittel = question phrase
• benutzt = finite verb
• du = subject

That is normal German word order for this kind of question.

Für means for and it takes the accusative case.

So:

• für dunkle Kleidung = for dark clothing

Even though English also uses for, in German it is useful to remember that für always governs the accusative.

Because dunkel is being used as an adjective before a noun, and in German adjectives in that position need an ending.

Here:

• Kleidung is feminine singular
• it comes after für, so it is accusative
• there is no article before it

That gives the adjective the ending -e:

• dunkle Kleidung

So dunkel Kleidung is incorrect because the adjective is missing its required ending.

Because Kleidung is being used in a general, uncountable sense, like clothing in English.

German often leaves out the article in this kind of expression:

• für dunkle Kleidung
• für helle Kleidung
• für empfindliche Wäsche

Without an article, the adjective has to carry more grammatical information itself, which is why you see dunkle.

Because Kleidung is a singular collective noun in German.

So:

• die Kleidung = clothing / clothes

English often uses the plural clothes, but German commonly uses the singular Kleidung for the general idea.

If you want to talk about individual items of clothing, you might use:

• Kleidungsstücke = garments / items of clothing
• Sachen or Klamotten = clothes, more informal

Yes. Waschmittel usually means laundry detergent.

It does not normally mean just any soap or cleaning product.

Related words:

• Waschmittel = detergent for washing clothes
• Spülmittel = dish soap / washing-up liquid
• Seife = soap
• Waschpulver = washing powder
• Flüssigwaschmittel = liquid detergent

So in this sentence, Waschmittel is exactly the right word.

Yes, benutzen is completely natural.

Other possibilities are:

• verwenden = to use, a bit more formal
• nehmen = to use/take, often more conversational

For example:

• Welches Waschmittel verwendest du für dunkle Kleidung?
• Welches Waschmittel nimmst du für dunkle Kleidung?

All three can work, but benutzen is clear and standard.

Yes, but the meaning changes.

• dunkle Kleidung = dark clothing in general
• schwarze Kleidung = specifically black clothing

So if you mean all dark colors, dunkle Kleidung is better. If you mean only black clothes, schwarze Kleidung is more precise.

You would use Sie instead of du, and the verb changes to match:

Welches Waschmittel benutzen Sie für dunkle Kleidung?

So:

• du benutzt = informal singular
• Sie benutzen = formal singular or plural

Usually it asks which detergent the other person uses or recommends for that purpose.

Because of welches Waschmittel, it often sounds like:

• Which detergent do you use?
• possibly meaning which brand/type/product

It does not have to mean only one exact product forever; it just asks the listener to identify the detergent they use for dark clothes.