See the question and my original answer on StackOverflow

The FORM is treated specially by default by the Html Agility Pack. See here why: HtmlAgilityPack -- Does <form> close itself for some reason?

This code should get all the FORM elements:

HtmlDocument doc = new HtmlDocument();
HtmlNode.ElementsFlags.Remove("form");
doc.Load(myTestHtm);

foreach (var v in doc.DocumentNode.SelectNodes("//form"))
{
    Console.WriteLine(v.OuterHtml);
}