Question

How to change active voice into passive voice and passive voice into active voice ???

Answer 1

It's simple you just need to exchange the subject and object and do some modifications in the verb.
Ex- The shopkeeper is selling pencils. (Active)
Pencils are being sold by the shopkeeper. (Passive)

Answer 2

Let u see
Active : I eat a mango.
Passive:A mango is eaten by me.
First of all the object is main subject of passive verb and the subject of active verb is made subject of passive verb or a preposition.
I hope this answer is right