아세요>알아

Because ㄹ should be eliminated to avoid the clash between ㄹ and one of ㄴ, ㅂ, ㅅ, 오. It's a rule, which is called "ㄹ 탈락 규칙". You need to keep in mind since you will meet some more verbs like 알다, 살다, 돌다, 울다 etc. 어디 Where + 살 (excluding 다) + -세요 ( ㄹ meets ㅅ, remove ㄹ) → 어디 사세요? 어디 + 살 + -는 + 애야 (ㄹ meets ㄴ, remove ㄹ) → 어디 사는 애야? 어디 + 살 + -십니까 ( ㄹ meets ㅅ, remove ㄹ) → 어디 사십니까? (Formal to speak) About -(으)세요, you can use this form when you ask someone something using the honorific, or when you talk about someone you shouldn't talk down to. There're two ways to use the honorific, lowering yourself and exalting someone. So 아세요 can't mean "I know" as you exalt yourself in it. The meaning it carries would be "S/he knows" after all. 알아요 ＞ 알아 바쁘세요? ＞ 바빠요? ＞ 바빠? 식사하세요 ＞ 밥 먹으세요 ＞ 밥 먹어요 ＞ 밥 먹어 공부하세요? ＞ 공부해요? ＞ 공부해? 쓰지 마세요 ＞ 쓰지 마요 ＞ 쓰지 마 I don't think I've said 밥 먹으세요 except for when I was with children. When you're going to say to old people over 60's, you can say 진지 드세요(most honorific).