why is this happening? Model file is from The Model's Resource, Nintendo model archive
*Edited after seeing comments(thanks!) I saw your comment and reply, and watched some videos about bsdf, still I got a problem as you can see my pictures below. I somehow erased the shadows, but the eyes became too tranparent and also are looking weird.
I only changed the eyes, not the mouth at all. so the question is on eyes, at this point.