1) Linear and nonlinear first order differential equations differ in their theory of existence and uniqueness of solutions, properties of general solutions, and whether solutions can be expressed explicitly or only implicitly. 2) For linear equations, solutions are guaranteed to exist and be unique based on continuity of coefficients, and general solutions involving arbitrary constants can represent all solutions. 3) Nonlinear equations may only have local existence and uniqueness depending on initial conditions, general solutions may not encompass all solutions, and solutions are often only implicitly defined requiring numerical methods.