Molecules having one lone pair and three bond pairs have geometrical shape

Explanation

Four regions of electron density arrange themselves tetrahedrally, but only the three bonded atoms are visible, so the shape is a pyramid, as in ammonia with its bond angle of about 107 degrees. Trigonal planar at 120 degrees would require three bond pairs and no lone pair, as in boron trifluoride, which is the trap here. A bent shape comes from two bond pairs with two lone pairs, as in water.