Learn Swedish: Vad har ni för rosor idag? - What kind of roses do you have today?

This is the standard way to use the split interrogative vad ... för (what kind of). In a Swedish main clause, the verb (har) must come in the second position, right after the question word vad. The subject (ni) follows the verb, and then we place för right before the noun it modifies (rosor).

Can I keep vad för together at the beginning of the sentence?

Yes, you could technically say Vad för rosor har ni idag? However, splitting the phrase (Vad har ni för...) is much more common and sounds significantly more natural in everyday Swedish.

Why is the noun rosor (roses) in the indefinite form?

The phrase vad ... för asks about the type, category, or quality of something, so it always requires an indefinite noun. You are asking about the kind of roses in general, not asking about specific, already-identified roses.

Why use the plural pronoun ni if I am just talking to one florist?

When talking to a store employee, Swedes often use the plural ni to refer to the business or the staff as a whole (similar to saying "what kind of roses do you guys have?" in English). It is not the old-fashioned formal ni, but rather a collective plural "you" for the shop.