Wow,, It is really hard to explain even for me
But I've found the right answer.!
First, -아/어 있다 is used with verbs like(앉다-sit, 서다-stand, 떨어지다-fall)
---> 앉아 있다, 서 있다, 떨어져 있다 These verbs mean the situation is remained in completed situation(It CAN be completed by one move)
Second, -고 있다 is used with verbs like(설치하다-install-put, 자다-sleep, 공부하다-study, 떨어지다-fall)
-->설치하고 있다, 자고 있다, 공부하고 있따, 떨어지고 있다 These verbs mean the situation is still being in the process of the verbs.(It CAN'T be completed by one move)
Third, Some verbs can be used with both of -아/어 있다 and -고 있다
As I mentioned earlier the "떨어지다"!!!! THIS is an exception. Imagine 2 situations, 1. an apple have fallen on the floor= lay on the floor(사과가 바닥에 떨어져 있다) and 2. an apple is falling from the tree (사과가 나무에서 떨어지고 있다).
SO except the verbs like "떨어지다-fall", the First verbs and Second verbs have its right positions. It sounds weird, when they are used with different form.
Do you understand? :) Thank you for reading