"He is not really perfect."
Translation:Anh ấy không thật sự hoàn hảo.
oh it is, I'm sorry I might confuse you with the translation really. "thật" alone can also be an adverb, but it bears the meaning of really, very as an intensity, to add emphasis to the adjective or adverb. on the other hand, "thật sự" means completely, wholly, fully.
Yes, it is true, we were taught earlier we just needed 'thật' to mean 'really x' or 'really y'.
If 'thật sự' just adds intensity to the meaning, I would suggest that just 'thật' be added as an acceptable answer. In English to add the emphasis we would often say 'completely, totally,' etc in combination or even "really, really, really" in combination.