When do you want the validation to happen? When you click the submit button?
An easy way would be to have a button(type="button") and an onclick event:
<input type="button" value="Click" onclick="validate();" />
Then define a
validate function, inside it use javascript to take the values of the fields you want to validate and if successful submit the form.
You will find good tutorials here:
http://www.w3schools.com/default.asp
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