From what I understand:
Preprocessor directive includes files that already contain code and can help support your programme.
Main function is the main body in which the programme runs.
Expression Statements allows the user to create expressions, such as code that print out text or even change a variable.
Declaration statement declares variable that can be altered later on if they are not a const.
Return statement will return the result a user is looking for, in the case for a return stament in the main function, it will most likely end the programme.