"초등학교에 간 후에 중학교에 가세요."
Translation:After going to the elementary school, go to the middle school.
Main clause: 중학교에 가세요 (Go to middle school)
Sub clause: 초등학교에 가- (to go to elementary school)
Time conjunction: 후에 (after)
후에 dictates that for the main clause to take place, the sub clause has to be completed. Hence, the verb of sub clause in a 후에-sentence will always be conjugated in the past [relative verb] : "past to what is to take place in main clause [main verb]"
후에-sub clause: 초등학교에 간 후에
Alright, this one has me stumped. What is 간? Going is 가는, 간 should be gone or 間(간) meaning interval, except it's neither. It's ～(으)ㄴ 후에 which for some unknown reason everyone's taken to translating as "after ~ing" in English. In the Japanese explanation though it doesn't look like that. It looks like "After you go to the elementary school, go to the middle school." This makes more sense in English to me, and looks closer to the Korean . . .
My answer "After going to an elementary school, go to the middle school" was accepted, but I received a comment "you have a typo, it should be "After going to a elementary school, go to the middle school".
Obviously the comment misses the mark. The article should be "an", not "a".