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HTML 

              
                <div class="container">
	<div class="row">
		<div class="form-box">
        <h1>Form <span>Registration</span></h1>
    	    <form role="form" id="contact-form">
            <!-- name field -->
                <div class="form-group">
                <div id="nameError" class="sr-only" role="alert"></div>
                <label for="form-name-field" class="sr-only">Name</label>
                    <div class="input-group">
                        <div class="input-group-addon"><span class="glyphicon glyphicon-user"></span></div>
                        <input type="text" class="form-control" id="form-name-field" value="" placeholder="Name">
                    </div>
                </div>
            <!-- email field -->
                <div class="form-group">
                <div id="emailError" class="sr-only"" role="alert"></div>
                <label for="form-email-field" class="sr-only">Email</label>
                    <div class="input-group">
                        <div class="input-group-addon"><span class="glyphicon glyphicon-envelope"></span></div>
                        <input type="email" class="form-control" id="form-email-field" value="" placeholder="Email">
                    </div>
                </div>
            <!-- phone field -->
                <div class="form-group">
                <div id="phoneError" class="sr-only"" role="alert"></div>
                <label for="form-phone-field" class="sr-only">Phone</label>
                    <div class="input-group">
                        <div class="input-group-addon"><span class="glyphicon glyphicon-phone"></span></div>
                        <input type="tel" class="form-control" id="form-phone-field" value="" placeholder="Phone">
                    </div>
                </div>
            <div class="checkbox">
                <label>
                    <input type="checkbox" id="checkbox-form"> I agree to terms and condition.
                </label>
              <div id="term-field" class="well">
                <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aenean ut orci quam. Nam vel ligula et mi commodo tincidunt. Cras mi ex, mollis eu magna eu, iaculis dignissim sapien. Donec et arcu malesuada, molestie erat in, tempor quam.</p>
              </div>
            </div>
            <button type="submit" class="btn btn-default">Submit</button>
    	    </form>   
		</div>
	</div>
</div>
!

CSS 

              
                @import "compass/css3";

body {
    background-color: lightblue;
}

.form-box {
    width: 50%;
    margin: auto;
    padding-top:20px;
}

.form-box h1 {
    text-align: center;
    font-weight: bold;
    text-transform: uppercase;
    color: tomato;
}

.form-box h1 span {
    font-weight: lighter;
}
!

JS 

              
                document.getElementById("contact-form").onsubmit = function() {
    //Validate Name
    if(document.getElementById("form-name-field").value === "") {
        document.getElementById("nameError").innerHTML = "Invalid Name! Please try again.";
        document.getElementById("nameError").className = "alert alert-danger";
        document.getElementById("nameError").style.display = "block";
    }
  //Validate Email
   if(document.getElementById("form-email-field").value === "") {
        document.getElementById("emailError").innerHTML = "Invalid Email! Please try again.";
        document.getElementById("emailError").className = "alert alert-danger";
        document.getElementById("emailError").style.display = "block";
    }
  //Validate Phone
   if(document.getElementById("form-email-field").value == "") {
        document.getElementById("phoneError").innerHTML = "Invalid Phone Number! Please try again.";
        document.getElementById("phoneError").className = "alert alert-danger";
        document.getElementById("phoneError").style.display = "block";
    }
 }
document.getElementById("term-field").style.display = "none";
document.getElementById("checkbox-form").onclick = function() {
  if(document.getElementById("checkbox-form").checked) {
    document.getElementById("term-field").style.display = "block";
  } else {
    document.getElementById("term-field").style.display = "none";
  }
}
!
999px

Console