What are the main postulates of Valence Shell Electron Pair Repulsion (VSEPR) theory? What improvement was made by Nyholm and Gillespie?

Predict the shapes of the following molecule using the valence shell electron pair repulsion model.

${\mathrm{SiF}}_4$

Predict the shapes of the following molecule using the valence shell electron pair repulsion model.

${\mathrm{BF}}_3$

Predict the shape of the following molecule using the valence shell electron pair repulsion model.

${\mathrm{NH}}_3$