Question

How does transparent object form shadow?

Answer 1

The object must be opaque or translucent to make a shadow. A transparent object will not make any shadow, as light will pass straight through it.
A shadow is made when an object blocks light. The object must be opaque or translucent to make a shadow. A transparent object will not make any shadow, as light will pass straight through it.

And Ohh Yes, there is no known material on earth that can transfer light without some minuscule losses.
Transparency is a property that allows light to pass through something. Like many people have said, nothing is completely transparent, so there will always be some shadow. Basically, the better you can see through something, the more transparent it is, and the lighter the shadow will be (because more light is passing through and not being blocked).