<script>
$(document).ready(function(){
    $("#myform").validate({
  groups: {
    username: "fname lname"
  },
  errorPlacement: function(error, element) {
     if (element.attr("name") == "fname" 
                 || element.attr("name") == "lname" )
       error.insertAfter("#lastname");
     else
       error.insertAfter(element);
   },
   rules: {
	   fname:{
		   required:{
			   depends: function(element){
				   var status = false;
				   if( $("#yesname:checked").val() !== undefined){
					   var status = true;
				   }
				   //console.log("yesname is: "+status);
				   return status;
			   }
		   }
	   },
	   lname:{
		   required:{
			   depends: function(element){
				   var status = false;
				   if( $("#yesname:checked").val() !== undefined){
					   var status = true;
				   }
				   //console.log("yesname is: "+status);
				   return status;
			   }
		   }
	   }
   },
   debug:true
 })
  });
</script>
  <form id="myform">
  <label for="name">Do you have a name?</label><br>
  	<input type="checkbox" name="name" value="y" id="yesname">yes<br>
    <input type="checkbox" name="name" value="n" id="noname">no<br>
    <p>if you have a name give it to me!</p>
	<label>Your Name</label>
	<input name="fname" id="firstname" placeholder="first name..." />
	<input name="lname" id="lastname" placeholder="last name..." />
	<br/>
	<input type="submit" value="Submit"/>
</form>

http://bassistance.de/jquery-plugins/jquery-plugin-validation/

Conditional JQuery Validate Required Field

jquery

jQuery