Learn German: Wofür kaufst du den Stoff? Für ein schönes Kleid? - What are you buying the fabric for? For a beautiful dress?

In German, when you ask 'for what' about a thing or a concept, you combine wo- and the preposition (here, für). You will hear für was in casual everyday speech, but wofür is the correct standard form you should use and write.

Could I use Wofür to ask who the fabric is for?

No, wo- compounds are only used for inanimate objects and ideas. To ask about a person, you use the preposition plus the question word for people. Since für always requires the accusative case, you would ask: Für wen kaufst du den Stoff? (For whom...).

Why is it den Stoff instead of der Stoff?

Stoff (fabric) is a masculine noun. In this sentence, it is the direct object being bought, so it must be in the accusative case. This changes the masculine article from der to den.

Why does the adjective end in -es in ein schönes Kleid?

The preposition für always forces the noun that follows into the accusative case. Kleid (dress) is a neuter noun, so its indefinite article is ein. Because ein doesn't clearly show the neuter gender on its own, the adjective does the heavy lifting by taking the -es ending.