"In short, you understood !"
Translation:Bref, vous avez compris !
Is there any case in which "t'es" could be accepted? Or is it always written "tu es"?
In standard French grammar, no, there would never be an instance that you'd end up saying t'es. However, like English speakers, French speakers don't always follow all their own grammatical rules in casual conversation, and I'm pretty sure that shortening tu to "t' is one of the "mistakes" that's commonly heard in informal settings.