Sometimes you want the user to be able to tick a box.  Add this code to your form in forms.html just before the submit button:

<div>
    <label for="approval">Keep me logged in: </label>
    <input type="checkbox" value="yes" name="cookie" id="cookie">
</div>

Save and try. 

Now add the attribute checked inside the input tag just after all the other attributes.  No value is needed for that attribute.