Html.Form returns a Form object (in our case we have two kinds - Simple and Mvc<>) and not a string, so using the "<%=" (which is sugar for Response.Write) outputs the tags AND the type.

In this case you want to do one of two things:

<%using(Html.Form(...)){%>

<%}%>

or

<form action="<%=Url.Action("action","controller")%>">