When I open my sink faucet, very little water passes through. However, the sink spray has full power. (The amount of water that passes has decreased over time. We have some sketchy water pressure at times, so I thought that might be the problem, but it's not.) How can I remedy this without calling a plumber?

We neede the brand name of the faucet. Also the type. One handle or two knobs? How old is it? Have you unscrewed the aerator from the spout and checked and cleaned the screens? Turn the water on after you remove the aerator and see how the pressure is now. Regards, tom