when do we use es & when do we use esta like es muy amable why not esta with accent on top of a?

They are both correct in that they are the third person conjugation of Ser and Estar. The difference however is that the use of es/ser suggests that the subject is always or inherently kind. The use of está/estar suggests that the subject is only kind on this occasion. It can be also explained with the example "The apple is green". If it is a green variety and therefore always green then you would use es/ser. If it was green from being unripe then you would use está/estar.