A modular building system for constructing the frame of a structure. Standardized foundation forms, vertical forms, and tie beam forms are attached to each other. The vertical forms are hinged so as to be capable of defining a corner of any angle. Cover plates are selectively inserted into the tie beam forms so as to define a reception recess which corresponds to the size of a roof truss being used. The various forms can be attached to each other with a minimal amount of labor.