using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        string naam = "van der van de iets jantjevander van huizen";

        string[] middlenaam = new[] { "der", "de", "van", "iets" };

        var resultaat = string.Join(' ', naam.Split(' ').SkipWhile(x => middlenaam.Contains(x)));

        Console.WriteLine(resultaat);

    }

}