I have an american standard bathtub faucet. It was leaking so I changed the washers,now it is to tight to turn. What did I do wrong?

You probably tighten the bonnet nut to tight. The bonnet nut is the nut around the stem that prevents water from leaking out around the stem. The tighter you tighten it, the more it compresses the gasket around the stem to prevent leaking. Loosen it until it starts leaking and the retighten until leaking stops.