我在.net为何不能用到多个表单..各位帮帮忙..

<body>
<form id="frmdg" runat="server">
<center><asp:DataGrid  ID="dataGrid1" AutoGenerateColumns="False"  AllowPaging="true" Font-Size="10" PageSize="10" PagerStyle-BackColor="#F5FFC1" PagerStyle-Mode="numericPages" PagerStyle-HorizontalAlign="center" OnPageIndexChanged="Page_Change" AlternatingItemStyle-BackColor="#EFF7FF"
        OnEditCommand="MyDataGrid_Edit"
OnCancelCommand="MyDataGrid_Cancel"
        OnUpdateCommand="MyDataGrid_Update"
OnDeleteCommand="datagrid1_delete"
 runat="server">
 <HeaderStyle HorizontalAlign="Center" ForeColor="White" 
 BorderColor="#6876C5" BackColor="#6C88D9"></HeaderStyle>
 <columns>
 <asp:EditCommandColumn
EditText="编辑"
CancelText="取消"
UpdateText="更新"
ItemStyle-Wrap="false"
HeaderText="编辑列"
HeaderStyle-Wrap="false"
/>
 <asp:boundColumn HeaderText="索引" 
 ItemStyle-Width="20" HeaderStyle-Width="30" ReadOnly="true"  datafield="Id"/>
 <asp:boundColumn HeaderText="用户名" datafield="username" />
 <asp:boundColumn Headertext="日期" ItemStyle-Width="120" DataField="Tdate" />
 <asp:boundColumn Headertext="工程代码" ItemStyle-Width="50" ReadOnly="true" DataField="Pcode" />
 <asp:boundColumn Headertext="活动代码" ItemStyle-Width="60" ReadOnly="true" DataField="Acode" />
 <asp:boundColumn Headertext="内容" ItemStyle-Width="200" ReadOnly="true" DataField="Content" />
 <asp:boundColumn Headertext="计划时间" ItemStyle-Width="30" ReadOnly="true" DataField="PlanTime" />
 <asp:boundColumn Headertext="实际时间" ItemStyle-Width="30" ReadOnly="true" DataField="pracTime" />
 <asp:boundColumn Headertext="出差地点" ItemStyle-Width="50" ReadOnly="true" DataField="evecSpot" />
 <asp:boundColumn Headertext="出差天数" ItemStyle-Width="30" ReadOnly="true" DataField="evecdays" />
 <asp:boundColumn Headertext="费用" ItemStyle-Width="80" ReadOnly="true" DataField="charge" />
 <asp:ButtonColumn Headertext="删除列" Text= "删除" CommandName="Delete"></asp:ButtonColumn> 
 </columns>
  <PagerStyle HorizontalAlign="Center" ForeColor="Black" Position="Top" BackColor="White" Mode="NumericPages"></PagerStyle>
 </asp:DataGrid>
 </center>
</form>
<form id="frmtxt" runat="server">
<table width="547" border="0" align="center">
  <tr>
    <td width="168" height="27" bgcolor="#FFCC33"><div align="center" class="style2">日期(yyyy/mm/dd)</div></td>
    <td width="369" bgcolor="#EFEFCF"> <div align="left"><span class="style3">
      <asp:TextBox ID="Last_Update_Date" TextMode="SingleLine" height="20" width=200 maxlength="19" readonly runat="server" />                    
          &nbsp;&nbsp;&nbsp;&nbsp;<input type="button" name="change2" value="展开"  onClick="show_cele_date(change2,'','',Last_Update_Date)">
    </span></div></td>
  </tr>
    <tr>
      <td height="28" bgcolor="#FFCC33">
<div align="center" class="style2">项目名称</div></td>
    <td bgcolor="#EFEFCF"><asp:DropDownList ID="DDListPcode" Height="20"  AutoPostBack="true" runat="server">
                      </asp:DropDownList>
    </td>
  </tr>
  <tr>
    <td height="24" bgcolor="#FFCC33"><div align="center" class="style2">活动<span class="style2">名称</span></div></td>
    <td bgcolor="#EFEFCF"><span class="style3">
      <asp:DropDownList ID="DDListAcode" Height="20"  runat="server" />      
    </span></td>
  </tr>
  <tr>
    <td height="23" bgcolor="#FFCC33"><div align="center" class="style2">计划时间(小时)</div></td>
    <td bgcolor="#EFEFCF"><span class="style3">
      <asp:TextBox ID="txtPlantime" Height="20" runat="server" />
  <asp:RequiredFieldValidator ID="RegValidPtime" ControlToValidate="txtPlantime" ErrorMessage="请输入计划时间" 
  runat="server" />  
  <asp:RangeValidator id="RangvaliPTime"
  ControlToValidate="txtPlantime" forecolor="red" MinimumValue="0" MaximumValue="8" type="Double"  Display="Dynamic"
  ErrorMessage="必需是介于0到8的数字" runat="server"
  />
    </span></td>
  </tr>
  <tr>
    <td height="28" bgcolor="#FFCC33"><div align="center" class="style2">实际时间(小时)</div></td>
    <td bgcolor="#EFEFCF"><span class="style3">
      <asp:TextBox ID="txtpariTime" Height="20" runat="server" />
  <asp:RangeValidator id="RangvaliNTime"
  ControlToValidate="txtpariTime" forecolor="red" MinimumValue="0" MaximumValue="8" type="Double"  Display="Dynamic"
  ErrorMessage="必需是介于0到8的数字" runat="server"
  />
    </span></td>
  </tr>
  <tr>
    <td height="29" bgcolor="#FFCC33"><div align="center" class="style2">出差地点 </div></td>
    <td bgcolor="#EFEFCF"><span class="style3">
      <asp:TextBox ID="txtevecSpot" Height="20" runat="server" />      
      <asp:Label ForeColor="#FF0000" ID="Lbldays" runat="server" />      
</span></td>
  </tr>
  <tr>
    <td height="30" bgcolor="#FFCC33"><div align="center" class="style2">出差天数</div></td>
    <td bgcolor="#EFEFCF"><span class="style3">
      <asp:TextBox ID="txtevecdays" Height="20" runat="server" />      
</span></td>
  </tr>
  <tr>
    <td height="28" bgcolor="#FFCC33"><div align="center" class="style2">作业费用(元)</div></td>
    <td bgcolor="#EFEFCF"><span class="style3">
      <asp:TextBox ID="txtprice" Height="20" runat="server" />
    </span></td>
  </tr>
  <tr>
    <td height="24" bgcolor="#FFCC33"><div align="center" class="style2">作业内容描述</div></td>
    <td bgcolor="#EFEFCF"><span class="style3">
      <asp:TextBox ID="txtcontent" TextMode="MultiLine" Width="400" Height="20" runat="server" />
    </span> </td>
  </tr>
</table>
<table width="547" border="1" align="center">
  <tr bgcolor="#6C88D9">
    <td height="42" bgcolor="#6C88D9"><div align="center">
      <asp:Button ID="ButtonAdd" Width="60" Text="添加" OnClick="btAdd_Click" runat="server" />      
    </div></td>
    <td ><div align="center">
      <asp:Button ID="ButtonSave"  width="60" Text="取消" onClick="btSave_Click" runat="server" />    
    </div></td>
    <td ><div align="center">
      <asp:Button ID="ButtonBack"  width="60" Text="返回" onClick="btBack_Click" runat="server" />    
    </div></td>
  </tr>
</table>
</form>
</body>
如上的两个表单..怎么运行时会显示在一个页面只能有一个表单的错误...大家帮帮忙啦..最好有个例子说明...重谢..!!