Sample pages which can be used to understand HTML forms are:
Basic form elements
Text input
Password input
Radio button
Check box
Drop down box
Select List
Textarea
Button
Basic form elements with some default value:
Text input - 2
Radio button - 2
Check box - 2
Drop down box - 2
Select List - 2
Textarea - 2
Extra form elements:
Label
Field set
Optgroup
Optgroup - 2
Frames
Horizontal frames
Vertical frames
Horizontal frames without border
Horizontal frames with fixed border
Horizontal frames opening other websites
Noframes example
Mixed frames
iframe