Hi! It is true, the 여요 ending is only used with 하다 verbs, and it becomes 하여요, but overtime koreans started to say 해요 instead of 하여요, and I think 하여요 is no longer used. =)

Hi ! Verbs end with (하다) are conjugated by using ( 여요) at the end of the verb after removing the (다) part . But then after it turns to be (해요 ). For Example : 공부하다 --> 공부하여요 ---> 공해요 ( to study ) so it ends up by saying that : verbs end with (하다) .. changes to (verb stem + 해요) .. to facilitate pronunciation :D HOPE IT HELPED =)