문득 means suddenly, it's an adverb. ㄴ/는 셈이다 means "it is the same as" or "it is like" it gets complex using that in a sentence. 제밥이다 means be better then expected . Hope that helped!

제법(이다) and 문득, ㄴ/는 셈이다 * 제법(adv) - indicates that some skill or quality is up to a formidable level. Up to scratch; Up to snuff. 제법이다 is the adjective form. E.g. - 날씨가 제법 춥다 = It is quite cold (more than I expected). - 음식 솜씨가 제법인 친구 = A friend with a surprisingly good cooking abilities. - 반년 배운 기타 솜씨로는 제법이다 = Your guitar playing is quite good for the six months you put into it. * 문득(adv) - suddenly. It is always used for a thought or feeling, or something that caused it. E.g. - 하늘을 보다가 문득 고향 모습이 떠올랐다 = While looking at the sky, I suddenly thought of the scenes of my old home. - 문득 생각난 그떄 그 사람 = The one in those days that suddenly came to my mind. * -ㄴ/는 셈이다 - It comes/boils down to (the fact that) ...; It effectively means ... 셈 means "computation" or "figuring", so -는 셈이다 originally comes from "if you figure it, it comes to ...". - 비용과 매상을 다 따져 보니 본전 장사인 셈이었다 = Accounting for all the costs and receipts, it came to a brake even point.