완성(n) is the root word of 완성하다 (verb) = to complete. 완전(n) is the root word of 완전하다 (adj) = complete, perfect. 완전 as a noun is not used much by itself. * 완전 is often used as an adverb to mean 완전히 (completely, totally). This has become popular recently. - 건물의 완성 = completion of a building. - 건물을 완성했다 = I/They completed the building. - 완전한 승리 = a complete victory (완전한 = adjective). - 이번 일은 완전(히) 실패했다 = We totally failed on this one. (완전(히) = adverb)