Form Tags In ASPX Pages

have working ASP and HTML pages that use Form methods such as GET to obtain data from an external site and show it in the page.
When I try to use this method in an aspx page, I get an error saying that only one server-side form can be used - the one that encloses all the HTML code and uses the POST method.
Is there a way to adapt the ASP technique to use it in an aspx page, and how is it done?
Thanks.

Can you give us an example of what you are talking about?

you can't add another form tag to an aspx page - well, that's not strictly true, but it's very tricky. If you're getting input from the user, encapsulate that form in a user control that can handle its own postbacks. If you're talking about something else, I'm not understanding you