interface_abstract_class.htm
Abstract class is an incomplete class, which is expected to be changed down the road and must be inherited by a class and a method override method will do the job 
    //csc interface_abstract.cs
using System;

namespace soccer
{
	interface team
	{
		string method_1(string str);
		string method_2(string str);
	}
//abstract class A : team
//note the error when a method is abstract and class in not an abstract
 class A : team
	{
		
public abstract string method_1(string str);
		public abstract string method_2(string str);
		

	}
	class B : A
	{
     public override string method_1(string str)
     { Console.WriteLine(" Hello : " + str); return str;}
	 public override string method_2(string str)
	 { Console.WriteLine(" Hello : " + str); return str ;}
	} 
	class test: B
	{
		public static void Main(string[] args)
		{
			Console.WriteLine("Player for team 1 : ");
			string s1 = Console.ReadLine();
			Console.WriteLine("Player for team 2 : ");
			string s2 = Console.ReadLine();
			B b = new B();
			b.method_1(s1);
			b.method_2(s2);
			
		}
	}
}
You must override the method
  
    //csc interface_abstract_1.cs
using System;

namespace soccer
{
	interface team
	{
		string method_1(string str);
		string method_2(string str);
	}
//abstract class A : team
//note the error when a method is abstract and class in not an abstract
 abstract class A : team
	{
		public abstract string method_1(string str);
		public abstract string method_2(string str);
	}
	class B : A
	{
	//public override string method_1(string str)
       { Console.WriteLine(" Hello : " + str); return str;}
	public string method_1(string str)
	{ Console.WriteLine(" Hello : " + str); return str;}
	public override string method_2(string str)
	{ Console.WriteLine(" Hello : " + str); return str ;}
	}
	class test: B
	{
		public static void Main(string[] args)
		{
		Console.WriteLine("Player for team 1 : ");
		string s1 = Console.ReadLine();
		Console.WriteLine("Player for team 2 : ");
		string s2 = Console.ReadLine();
		B b = new B();
		b.method_1(s1);
		b.method_2(s2);
			
		}
	}
}

now let us correct all the code and compile

    //csc interface_abstract.cs
using System;

namespace soccer
{
	interface team
	{
		string method_1(string str);
		string method_2(string str);
	}
abstract class A : team
	{
		public abstract string method_1(string str);
		public abstract string method_2(string str);
	}
	class B : A
	{
	public override string method_1(string str)
	{ Console.WriteLine(" Hello : " + str); return str;}
	public override string method_2(string str)
	{
	Console.WriteLine(" Hello : " + str); return str ;}
	}
	class test: B
	{
		public static void Main(string[] args)
		{
		Console.WriteLine("Player for team 1 : ");
		string s1 = Console.ReadLine();
		Console.WriteLine("Player for team 2 : ");
		string s2 = Console.ReadLine();
		B b = new B();
		b.method_1(s1);
		b.method_2(s2);
			
		}
	}
}
Now let us return the output from the method to the caller
    //csc interface_abstract_return.cs
using System;

namespace soccer
{
	interface team
	{
		string method_1(string str);
		string method_2(string str);
	}
abstract class A : team
	{
		public abstract string method_1(string str);
		public abstract string method_2(string str);
	}
	class B : A
	{
	public override string method_1(string str){  return str;}
	public override string method_2(string str){ return str ;}
	}
	class test: B
	{
	   public static void Main(string[] args)
		{
		Console.WriteLine("Player for team 1 : ");
		string s1 = Console.ReadLine();
		Console.WriteLine("Player for team 2 : ");
		string s2 = Console.ReadLine();
		B b = new B();
		Console.WriteLine(" Hello : " + b.method_1(s1));
		Console.WriteLine(" Hello : " + b.method_2(s2));
			
		}
	}
}