<input type="text" name="username" required>

$("#myForm").validate({
  rules: {
    username: {
      required: true
    },
    email: {
      required: true,
      email: true
    }
  }
});

<input type="text" name="username" class="required">

$("#myForm").validate();

$("#username").rules("add", {
  required: true,
  messages: {
    required: "用户名是必填项"
  }
});

$("#myForm").validate({
  rules: {
    username: {
      required: true
    }
  },
  messages: {
    username: {
      required: "请输入您的用户名"
    }
  }
});

$("#myForm").validate({
  rules: {
    email: {
      required: function() {
        return $("#newsletter").is(":checked");
      }
    }
  }
});

$("#myForm").validate({
  groups: {
    nameGroup: "firstName lastName"
  },
  errorPlacement: function(error, element) {
    if (element.attr("name") == "firstName" || element.attr("name") == "lastName") {
      error.insertAfter("#lastName");
    } else {
      error.insertAfter(element);
    }
  }
});