I must be getting my languages confused...

Here's what I want to do in C#

//create an interface
public interface IAdd
{
   public int moveXUnits(int XUnits);
}

//create an abstract base class that inherits the interface
public abstract class Movement : IAdd
{
   private int startUnit;

   public Movement()
   {
      startUnit = 0;
   }

   public Movement(int initializeUnit)
   {
      startUnit = initializeUnit;
   }

   public int moveXUnits(int XUnits)
   {
      startUnit += XUnits;
      return startUnit;
   }
}
 
//finally create some concrete classes that inherits from the abstract base class
public class Walk : Movement
{
   base(1);
   public void Step()
   {
      moveXUnits(2);
   }
}

public class Skip : Movement
{
   base(2);
   public void Step()
   {
      moveXUnits(3);
   }
}

public class Run:Movement
{
   base(3);
   public void Step()
   {
      moveXUnits(5);
   }
}

So my first question is: what is the analogous keyword for base in C#. Because 'base' is giving me an error.
Second question is: is this right? I have a feeling that it isn't.

What are those base() calls meant to do? Call the base class constructor? In that case you need to make the calls as part of the derived class constructors. Ex:

Walk() : base(1) {}

You're exactly right. On both points.

Though it's a simple example, any other improvements would be much appreciated.

   public interface IAdd
    {
        int moveXUnits(int XUnits);
    }

    public abstract class Movement : IAdd
    {
        private int startUnit;
        private int stepUnits;
        private int totalSteps;

        public Movement()
        {
            startUnit = 0;
            stepUnits = 1;
            totalSteps = 0;
        }

        public Movement(int initializeUnit, int initializeStep)
        {
            startUnit = initializeUnit;
            stepUnits = initializeStep;
            totalSteps = startUnit;
        }

        public int moveXUnits(int XUnits)
        {
            stepUnits = XUnits;
            totalSteps += stepUnits;
            return totalSteps;
        }

        private int autoMoveXUnits()
        {
            totalSteps += stepUnits;
            return totalSteps;
        }

        public virtual void Step(int XUnits)
        {
            moveXUnits(XUnits);
        }

        public virtual void Step()
        {
            autoMoveXUnits();
        }

        public virtual void displaySteps()
        {
            Console.WriteLine("I have moved " + totalSteps.ToString() + " units.");
        }
    }

    public class Walk : Movement
    {
        public Walk() : base(1,2){}
    }

    public class Skip : Movement
    {
        public Skip() : base(2,3){}
    }

    public class Run : Movement
    {
        public Run() : base(3,5){}
    }

The IAdd interface to me is kind of weird, you're modeling a behavior (imo a good thing when it comes to interfaces), but it feels like two of the same behaviors...one as an abstract class and the other as an interface (Step vs moveXUnit), which basically are two differently named ways of doing the same exact thing. To me, if you want to define behaviors like "Move along X axis", "Move along y axis", "Move along x/y", those would be individual interfaces that something like Movement would implement. In that case, what would "Step" mean? Just feels redundant and muddying behavior.

Simple example granted, but defining behaviors like that in this way is kind of restricting in my opinion too. You may be able to skip, run, and hop, but that's the manner of how you do it (e.g. speed and other influences like hopping up and down), those behaviors don't need to care where you're moving. So Movement to me sounds like it should model how you're moving, and it gets used by, or itself operates on another object (interface) that models where you're moving.

Please see my other thread to see why I did all this in the first place.