그전에 is a loose, colloquial word for "before". 그 is "that" as in 그것, but 그전 simply means "sometime before" without a particular reference point other than "now". There is a variant form which is 그 전에 (with a space, and said with more stress on 그). This means "before that" or "before doing that" referring to something that was just mentioned. 이전에 is a little different. It has two major uses: 1) similar to 그전에, but more formal and suitable in writing, 2) (in XX 이전에 form) before XX, with a specific reference point XX. 이 here doesn't mean "this" of 이것 but a hanja word 以 (comparatively / relatively). Unlike 그전, 이전 (and its antonym 이후) is used as often as a noun. - 그전에는/이전에는 서울 가려면 하루 종일 걸렸어 = Before, it used to take a whole day to go to Seoul. - 곧 식사할 건데 그 전에 할말이 있다 = We'll eat in a few, but before that I've got something to say. - 산업 혁명 이전에는 노동 생산성이 현저히 낮았다 = Productivity was much lower before the industrial revolution. - 세계사는 산업 혁명 이전과 이후로 나눌 수 있다 = World history can be divided into before and after the industrial revolution (이전 and 전 used as a noun).

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