I think i've asked this before but don't think I've gotten a response (might have been just in the reporting section. Anyways, "thật" means truth / real, I think. So this would be translated as "you are truly beautiful" or "you really are beautiful" But "you are really beautiful" is equivalent to "you are very beautiful" and I do not think that "thật" and "rất" mean the same thing. Or has the english use of "really" bled over into vietnamese?
Really + adj =rất
rất khỏe = very/really strong
Really+ Verb = Thật
ba mẹ tôi thật nói tôi có thể xem phim. My parents really say I can see the movie.