ASP NET How to show part of page depenting on some condition?

I would like to ask, how to can I change (show or hide) part of the same web page according to some condition, like in PHP? E.g. I would like to hide log-in formular if user has sent submit button and password was correct. I've seen this technique used in PHP and it looks like:

<?php 
    if (! $logged_in) {
        //<form> ... </form>
?>

//...some common content...

<?php
    else {
    //<p> ... </p>
?>

Is there any way in ASP.NET using C# how to 'recycle' pages like in PHP?

Thank you in advance.