Can we use multiple forms in a web page?

There can be multiple forms, with hacks.

It is indeed a shortcoming of WebForms. In ASP.NET MVC you can implement as many forms as you want (and it is valid & correct behavior of web pages).

The reason all server controls are placed inside <form> tag is to allow the WebForms engine to recognize them, load their values & save their values from/to the ViewState. Almost all infrastructure of control management in WebForms is based on the idea that a tag contains everything you access from the code-behind.

As pointed out, this is one of the shortcomings of WebForms. I do want to point out, additionally, that with cross-page posting and validation groups, you can typically reach your desired behavior (for most "multi-form" solutions).

Regarding the additional question: the <form runat="server"> is parsed as HtmlForm class behind the scenes, which inherits from HtmlControl like any other HTML element with runat="server".

Unlike any other HtmlControl though, there can exist only one instance per page and it does not appear in the toolbox as it's added automatically to every new Form you create, so it's quite pointless.