Core idea
Four electron regions surround the central atom. Three are bonding pairs and one is a lone pair. Their electron-region geometry is tetrahedral, but only atom positions define molecular shape, so the visible atoms form a trigonal pyramid.
Useful relation: AX₃E means one central atom A, three bonded atoms X, and one lone pair E. Lone pair–bond pair repulsion is stronger than bond pair–bond pair repulsion.