"Drink tea without sugar!"
Translation:Şekersiz çay için!
için is plural or formal imperative, yes. iç would be fine. ("için" also translates to "for"; they're homonyms, no relation other than form and pronunciation.)
"şekersiz çay" would be preferred here, as an adjective modifying tea since "şekersiz" is an adjective (rather than an adverb phrase in English, "without sugar.")
If you did say something like "çayını şekersiz iç!" you actually make "şekersiz" into an adverb, you're putting a lot of emphasis on HOW to drink your tea. It's more like saying "Why not try drinking your tea WITHOUT SUGAR for once!" rather than telling you what kind of tea to drink.
I'd like to know that too, please. :) Why can't you say "çay şekersiz iç" but can say "şekersiz çay iç"? Is it just a word order Thing? And (sorry to bother but still didn't get it) why can you put the tea in accusative although (to me) it doesn't seem to be a specific direct object in the translation?:)
The position of "şekersiz" wouldn't change. Just the meaning.
'Şekersiz çay iç" -- Drink (sugar-fee/sugarless) tea/Drink tea without sugar
'Şekersiz çayı iç' -- Drink the (sugar-free/sugarless) tea/Drinkthe tea without sugar
The accusative case is used to distinguish specific from general direct objects. This means that it only occurs on direct objects. You will never see it on a subject.