Learn Hardcore German: Auf der Fensterbank stehen nun Blumen, die angenehm duften. - There are now flowers on the windowsill that smell pleasant.

In German the preposition auf can govern either the accusative (for movement) or the dative (for location). Here we’re describing where the flowers are standing (no movement), so auf takes the dative. Fensterbank is a feminine noun, and the feminine dative article is der.

Why does the verb stehen come before the subject Blumen?

German main clauses follow the Verb-Second (V2) rule: exactly one element can occupy the first position, the finite verb must be second, and everything else follows. Since Auf der Fensterbank is fronted into first position, stehen is in second position, pushing the subject Blumen into third.

Why is there no article before Blumen?

When you introduce plural nouns in a general or indefinite sense (similar to “flowers” rather than “the flowers”), German often omits the article. You could say Auf der Fensterbank stehen nun die Blumen (the flowers), but dropping the article keeps it indefinite: “flowers are now standing…”.

What role does nun play, and why is it placed right after stehen?

nun means “now,” indicating a new or changed situation. In V2 word order, after the verb has gone into second position, temporal adverbs (time words) usually come next. That’s why you see stehen nun rather than nun stehen.

What does the relative clause die angenehm duften do, and why is die used there?

The clause die angenehm duften modifies Blumen, telling us which flowers. die is the plural nominative relative pronoun referring back to Blumen (plural). Inside the clause, die is the subject of duften (“to smell pleasantly”).

Why is angenehm not inflected in angenehm duften?

Here angenehm functions as an adverb modifying the verb duften (“to smell pleasantly”). Adverbs in German do not take adjective endings. If angenehm were used attributively before a noun, it would need an ending (e.g., angenehm duftende Blumen).

What exactly does duften mean, and how is it different from riechen?

duften means “to emit a pleasant scent” or simply “to smell good.” It implies a positive odor. riechen is more general: it can mean “to smell” (sense an odor) or “to smell like,” and may be used for pleasant or unpleasant scents depending on context.

Could I rephrase this sentence using an attributive participle—like auf der Fensterbank stehen nun angenehm duftende Blumen—and if so, what's the difference?

Yes, you can say:
Auf der Fensterbank stehen nun angenehm duftende Blumen.
Here angenehm duftende is an attributive participle phrase modifying Blumen, with the adjective duftende carrying the appropriate ending. The meaning remains the same; the difference is stylistic and often a matter of preference or register (the participle version is more concise).