Our basement windows have a gap 1/4" gap around frame that we need to seal. When we get heavy rainfall water leaks through. How do we seal this to prevent water from coming in again?

Use a silicone sealant, applied with a sealant gun (inexpensive). If the gap is deep, find some polystyrene backing rod, and press it into the joint first, until it is 1/4-inch behind the surface of the frame. Use a rod that is slightly bigger than the gap, in your case 3/8-inch diameter. The rod in back plus the nozzle of the tube of sealant will produce an hourglass-shaped bead of sealant, which is ideal to accomodate movement between the foundation and the frame.

Make sure you use backing rods infused with an additive to prevent termite burrow - untreated they can eat thru.