How can I change HTML tags to ASPX tags without changing the designs on my page?

all the forms i downloaded in css template were all html forms but what i did was to copy all the the codes in the page source then pasted it in a new web form so it will be in an .aspx form. the error says:

Server Error in '/' Application.
Compilation Error
Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately.

Compiler Error Message: BC30456: 'Button1_Click' is not a member of 'ASP.contacts_aspx'.

i tried adding manually the code for button1 in the cs page but it still gets the same error..

'Button1_Click' is not a member of 'ASP.contacts_aspx'.

if i double click the button, it should redirect me automatically to the cs page, right? and it will automatically show this

protected void Button1_Click(object sender, EventArgs e)
{

}

but what's happening is, whenever i double click the button, it just redirects me to the design codes and generates this one

Protected Sub Button1_Click(sender As Object, e As EventArgs)

End Sub

obviously i can't put codes on that one because it's on the source page of the design.. :(