To display the error individually, you must use the form_error ('username') function . And so that you get the value of the field being checked, use the set_value ('username') function .
For these two functions to work, you would have to set a rule for the "username" field in your controller. If you specify which validation rules apply to this field.
<?php echo form_error('username'); ?>
<input type="text" name="username" value="<?php echo set_value('username'); ?>">
Here is a simple tutorial on the Login form