c# abstract methods: internally public and virtual?

I think you are asking a different question than most people think (in other words it seems like you understand what abstract means).

You cannot declare a private abstract method - the compiler issues an error. Both of these classes will not compile:

class Foo
{
    private abstract void Bar();
}

class Baz
{
    // This one is implicitly private - just like any other 
    // method declared without an access modifier
    abstract void Bah();
}

The compiler is preventing you from declaring a useless method since a private abstract member cannot be used in a derived class and has no implementation (and therefore no use) to the declaring class.

It is important to note that the default access modifier applied to an abstract member by the compiler (if you do not specify one yourself) is still private just like it would be if the method was not abstract.

Abstract is just a way to say: "I am here, but no one has told me what I'm going to do yet." And since no one has implemented that member yet someone must do that. To do that you have to inherit that class, and override that member.

To be able to override something it has to be declared either abstract or virtual, and must at least be accessible to the inheritor, i.e. must be marked protected, internal or public.

Abstract methods cannot be private and are virtual. They must be at least protected.