when are electron geometry and molecular geometry the same and different?

Answer 1

They are the same only when the central atom has no lone pairs.
E.g. for BH3 the central atom forms 3 bonds and has no lone pairs thus the geometry of both the molecule and the electron pairs of the central atom are planar trigonal.
For NH3 you have a lone pair and 3 single bonds. Thus the electron configuration is that for 4 electron pairs (1 lone + 3 bond pairs; tetrahedral configuration), whereas for the molecule shape, the lone pair is not "visible", thus the shape is a trigonal pyramid.