ποταμῷ γὰρ οὐκ ἔστιν ἐμβῆναι δὶς τῷ αὐτῷ → it is impossible to step twice in the same river, you cannot step twice into the same rivers
accusative case of δωρεά as adverb; gratuitously (literally or figuratively): without a cause, freely, for naught, in vain.
