"Proč zívá tak hlasitě?"

[BarbaraWhi574572]

"Why does she yawn so loudly" is correct. You need the adverb here, not the adjective.

[BoneheadBass]

My take on this would be that while "loudly" is absolutely correct, the use of "loud" instead is now so common that it's become what I'd call "acceptably correct" as English continues to evolve. I think you'd almost always hear "loud" rather than "loudly," at least in the US.

[VladaFu]

https://english.stackexchange.com/questions/337662/loud-and-loudly-how-to-use-them