a page can have only one server-side form tag

Question

I am using master page and when I run this page, it shows the following error message:

a page can have only one server-side form tag

Answer 1

Sometime when you render the current page as shown in below code will generate the same error

StringWriter str_wrt = new StringWriter();
HtmlTextWriter html_wrt = new HtmlTextWriter(str_wrt);
Page.RenderControl(html_wrt);
String HTML = str_wrt.ToString();

so how can we sort it?