"Give me, please, a dress."
Translation:Дайте, будь ласка, сукню.
Сукню is the accusative singular of сукня. It's in the accusative because it's the direct object of дати. With verbs like buy or give, the object being given (or bought) is in the accusative. The recipients the objects are given to (or bought for) on the other hand, go in the dative (indirect object) case.