The main topics and subtopics covered in RS Aggarwal Class 12 Solutions Chapter 28 - Differential Equations are: Introduction to Differential Equations, Basic concepts, Order of a differential equation, Degree of a differential equation, General and Particular Solutions of a Differential Equation, Formation of a Differential Equation, Solutions for First Order, First Degree Differential Equations, Differential equations with variables that can be separated, and Linear Differential Equations.