OOPS :-
1) learn Constructors and must implement it other wise you are not able to answer it like make class inharit it then create object and in this way make multiple class and see the behaviour of constructors.
2) What is interface
3) What is Abstract Class
4) Keys like 'as','in',is etc
5) Boxing and Unboxing
6) Polymorphism
7) Overloding
ASP.NET :-
1) State Management
2) Exception Handling
3) Web.Config and Global.ascx
4) Functionality of Controls in .net