"Why are you sad today?"
Translation:Bugün neden üzgünsün?
Turkish has a more flexible word order than English in many cases, but adverbs and adverbial expressions of time almost always come at the beginning of a sentence and don't get inserted between parts of other phrases. "Neden üzgünsün" - why are your sad? "Bugün neden üzgünsün" - why are you sad today? It would sound unnatural to insert the modifier into the shorter phrase.