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오셨어요 = 오 “come” + 시 (honorific) + ㅓㅆ 었 (past) + 어 (plain)  + 요 (polite)
Make the past tense of the Polite Form (see Verb styles) column 2) by inserting ㅆ어 었어 before the 요 of the present Polite Form in column 2 of the table (so 먹어요 -> 먹었어요, 받아요 -> 받았어요, 줘요-> 줬어요, etc.).
Note what happens with honorific verbs ending in 시: 셨어요.
사실은 “in fact”. On its own 사실 means “a fact”, although further down you will also see it without 은 still meaning “in fact”.

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