It's very easy - the only thing you need to remember is that: - 아요 is used when verb stem ends withㅏ or ㅗ - 어요 is used in any other case, when a verb stem DOESN'T end withㅏ or ㅗ There's also -여요 which is used only with verb stem -하 and transforms it into -해요 Also ㅗ +ㅏ becomes 와 so 보다 --> 봐요 Good luck with learning Korean :)

Yes, the principle is: like sounding syllables tend to go together. The general guideline is as shown below, but there are many exceptions. 1. if the last syllable of the stem is in 아/오 group, it will take 아. 2. otherwise, it is 어. 3. if the last syllable is in "으" group and without 받침(bottom consonant), and if there is a syllable preceding it, go with the preceding syllable. ex) #1: 높다->높아, 작다->작아, 맞다->맞아, 보다->보아->봐, 꽂다->꽂아, 쫓다->쫓아, 살다->살아, 말다->말아/마, 팔다->팔아... #2: 크다->크어->커, 넓다->넓어, 춥다->추워, 길다->길어, 먹다->먹어, 때리다->때리어->때려, 갈구다->갈구어->갈궈, 끊다->끊어... #3: 아프다->아파, 슬프다->슬퍼, 오르다->올라, 누르다->눌러, 고르다->골라, 서투르다->서툴러...