How to validate a valid user in forms authentication

i have login functionality in website which is developed in forms authentication with membership and roles,
here is after entering the username and password click on the login button is going to validate the user connecting with database in button click event.
but here is no button click event after clicking on the button is postback and nothing done in page_load of login.aspx.cs
but in Masterpage the used 
bool authenticated = System.Web.HttpContext.Current.User.Identity.IsAuthenticated;
and after it is going when user credentials are correct it is redirecting to default.aspx and not valid then trowing the errors, but i didn't get where these functionality happen
where the data checking happen and where the redirecting code has managed nothing in login.aspx and masterpage.
i am new to forms authentication, 
can any one suggest me a best example to understand the forms authentication process