As far as I know : "σε" means "you" - accusative case. "εσένα" is also "you" accusative , but it's an additional form used for emphasis . "σε βλ'επω" = " (I) see you " "εσένα σε βλέπω" = "(I) see you" . Both sentences mean basically the same, but the second one has some extra emphasis (I see you, not someone else) . But it's incorrect to use "esena" alone ("εσένα βλέπω" would be incomplete).