i added a page with html code to a master page and i got that mistake when i launched the page. i wonder what i did wrong!??!
<%@ Page Title="" Language="C#" MasterPageFile="~/YourGuruMaster.master" AutoEventWireup="true" CodeFile="YourGuru.aspx.cs" Inherits="YourGuru" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">
<style type="text/css">
.style3
{
width: 114px;
}
.style4
{
width: 293px;
}
.style5
{
width: 126px;
}
</style>
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
<form id="form1" runat="server">
<div align="center">
<p>
<asp:TextBox ID="Question" runat="server" style="margin-left: 0px" Width="584px"></asp:TextBox>
</p>
</div>
<div align="center">
<table style="width: 553px">
<tr>
<td class="style3">
<asp:Button ID="Ask" runat="server" Text="שאל" Width="50px" />
</td>
<td class="style4">
<asp:DropDownList ID="DropDownList1" runat="server">
</asp:DropDownList>
     
<asp:DropDownList ID="DropDownList2" runat="server">
</asp:DropDownList>
     
<asp:DropDownList ID="DropDownList3" runat="server">
</asp:DropDownList>
</td>
<td class="style5">
<asp:Button ID="Answer" runat="server" Text="ענה" Width="50px" />
</td>
</tr>
</table>
</div>
</form>
</asp:Content>
ASP.NET Web Forms won't let you put more than one form tag in a page - your master page contains that form tag, so this:
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
<form id="form1" runat="server">
<div align="center">
will cause problems as it's trying to add a form tag. You don't need this form here anyways as the form tag in your master page is quite adequate.
<head runat="server">
<title></title>
</head>
<body style="direction:rtl;width:100%;background-color:#808080">
<form id="frmLevel2" runat="server">
<div dir="ltr" style="width:1000px;">
<asp:Button ID="btnExit" runat="server" OnClick="btnExit_Click" Text="خروج" />
</div>
<div>
<asp:GridView ID="GVLevel2" runat="server" Font-Names="Times New Roman" Font-Size="9pt" Width="700px" AllowPaging="True" BackColor="White" BorderColor="#E7E7FF" BorderStyle="None" BorderWidth="1px" CellPadding="3" GridLines="Horizontal">
<AlternatingRowStyle BackColor="#F7F7F7" />
<FooterStyle BackColor="#B5C7DE" ForeColor="#4A3C8C" />
<HeaderStyle BackColor="#4A3C8C" Font-Bold="True" ForeColor="#F7F7F7" />
<PagerStyle BackColor="#E7E7FF" ForeColor="#4A3C8C" HorizontalAlign="Right" />
<RowStyle BackColor="#E7E7FF" ForeColor="#4A3C8C" />
<SelectedRowStyle BackColor="#738A9C" Font-Bold="True" ForeColor="#F7F7F7" />
<SortedAscendingCellStyle BackColor="#F4F4FD" />
<SortedAscendingHeaderStyle BackColor="#5A4C9D" />
<SortedDescendingCellStyle BackColor="#D8D8F0" />
<SortedDescendingHeaderStyle BackColor="#3E3277" />
</asp:GridView>
</div>
<div>
<asp:Label ID="erorlabel" runat="server"></asp:Label>
<asp:Label ID="lblresult" runat="server"></asp:Label>
</div>
</form>
</body>
</html>
My Error is " A page can have only one server-side Form tag. " too
i don't have master page
来源：https://stackoverflow.com/questions/5842264/a-page-can-have-only-one-server-side-form-tag