so a protected member of a base class remains to be protected in a derived class and can therefore be derived again, if i understand this one correctly?

Answer 1

Yes, that is correct. For example, the following code will work:
using System;

class A {
protected int x=42;
}
class B : A {

}
class C : B {
public C() {
//Access the protected member
Console.WriteLine(this.x);
}
}

class Program {
static void Main() {
C c = new C();
//Outputs 42
}
}

Answer 2

Yes that's correct.

Answer 3

u can use protected class in other class by determine inheritance "class form : prclass" and u can use protected member in new class this have a diferent with private method

Answer 4

Yes, you are right

Answer 5

Yes you are correct, but can be inherited by another class

Answer 6

what is the difference between sealed and private