Variable: old
An array of sanitized variables POSTed from the previous request.
In certain forms – for example, {{ user:login_form }} – you may encounter validation errors that will require you to resubmit your the form.
Instead of making your users re-type everything, you may use {{ old:[field_name] }} tag to display their previously-entered input values.
When using the {{ user:login_form }} to login, upon entering incorrect credentials, you will be shown the same page. You can use the {{ old:[field_name] }} tags to maintain the values like so:
{{ user:login_form }}
    {{ if errors }}
        ...
    {{ /if }}
    <label>Username</label>
    <input type="text" name="username" value="{{ old:username }}" />
    <label>Password</label>
    <input type="password" name="password" value="{{ old:password }}" />
    <button>Log in</button>
{{ /user:login_form }}