when do we use abstract class and when do we use virtual keyword in C#?

Abstract classes are used when there is a need to implement functionality in some methods, but leave others to be implemented by other classes.
Virtual is used when a method has functionality but other classes may choose to use this or create their own.

For e.g (from msdn[^]).

abstract class WashingMachine
{
   public WashingMachine()
   {
      // Code to initialize the class goes here.
   }

   public virtual void Wash()
   {
      //Wash clothes
   }
   abstract public void Rinse(int loadSize);
   abstract public long Spin(int speed);
}

Here all machines was but spin and rinse is left for other classes to implement.

You use abstract classes when you want to leave some functionality to be implemented by the derived classes. For an example, consider the following code,

abstract class DrawingHelper
{
  // LoadModel and SaveToFile are the same regardless of the drawing API.
  void LoadModel(string fileName)
  {
    ...
    ...
  } 
  void SaveToFile(string fileName)
  {
    ...
    ...
  }
  // This method is specific to drawing API.
  abstract public void Draw(int x, int y, int z);
}

class OpenGLDrawingHelper : DrawingHelper
{
  public override void Draw(int x, int y, int z)
  {
    ...
    ...
  }
}

class DirectXDrawingHelper : DrawingHelper
{
  public override void Draw(int x, int y, int z)
  {
    ...
    ...
  }
}

Thus the method "Draw" differs according to the drawing API. Consider the following code,

DrawingHelper dHelper;
// This is a pseudocode. The enum APIChoice is imaginary. Assume that "choice" is changed by the user input in your UI.
if(choice == APIChoice.OpenGL)
{
  dHelper = new OpenGLDrawingHelper();
}
else if(choice == APIChoice.DirectX)
{
  dHelper = new DirectXDrawingHelper();
}
dHelper.LoadModel("D:\\car.b3d");
dHelper.Draw(100, 200, 300);

The main difference between an abstract and a virtual method is you can't implement an abstract method in the base class. But you can implement a virtual function in the base class.