There is no rule like "if there is ㅗ in the front then add 아요"
행복해요 > 행복해 + 요 > 행복하다 + 요 > 행복 + 하다 + 요.
해 is shorten form of 하다.
There is no word like 행복아요.
February 19, 2017
행복 is a noun. (happiness)
행복하다 is the adjective verb (to be happy)
As you can see, in Asian languages, there are many verbs and/or adjective-verbs derived from nouns whereas it's the opposite in western languages where nouns and/or adjectives are derived from verbs.
February 20, 2017
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