Well this can't be done with just HTML alone, it is actually a combination of HTML, Javascript and CSS (cascading style sheets).
What you will want to search for are the CSS attributes "visibility" and "display". You will also want to research the Javascript method "getElementById()"
Here is an example you can try yourself below. Just cut and paste the code and save it as an HTML page on your desktop.
CODE
<html>
<head>
<title>Show/Hide Elements</title>
<script type="text/javascript">
function showhide() {
// Get a reference to your form's id
var form = document.getElementById("myform");
// If it is hidden, show it, otherwise hide it.
if (form.style.visibility == "hidden") {
form.style.visibility = "visible";
}
else {
form.style.visibility = "hidden";
}
}
</script>
</head>
<body>
<form method="POST" action="" id="myform">
<textarea name="comments" rows="3" cols="45"></textarea>
</form>
<!-- Call the showhide() javascript function -->
<input type="button" name="showhide" value="Show/Hide" onclick="showhide()"/>
</body>
</html>
Clicking the button should hide and show the form above. The display attribute will tell the page if it should be included in the natural flow of the page or not. You can toggle this as "block" or "none".
So research what you see in this example and you should get an idea of what you need to do.
Enjoy!
"At DIC we be hiding and showing form code ninjas! Just some of our forms are too ugly to be seen in public."