Abstract keywords in C #

After learning the C # language, I should be familiar with the abstract keyword. I thought so at the beginning.
Believe it or not, I will test you.

Q: under what circumstances must I add the abstract keyword before the definition of a class? It is clear that it is * Yes.
My first response is that when you want a method, attribute, and event in this class to be abstract, you must add abstract modification before the class definition.

What else? Think about it.

Another case is that if you define a subclass, the parent class is modified with abstract. In this subclass, if you do not want to use abstract
If the modification methods, attributes, and time are all implemented on a regular basis, you must add the abstract keyword before the definition of the subclass.