Here are the correct forms of the bracketed words:

1. I know that boy is from a western country but I don't know his **nationality**.
2. The air quality today seems to be... than it was yesterday (bad) **worse**.
3. You should ask the **librarian** to help you find the book if you can't find it yourself.
4. Jane is the most... girl I know; she spends most of her time **studying**.
5. My cousin is learning some music instruments but she wants to learn **piano**.
6. Children today spend... time playing indoors (little) **less**.

If you need anything else, feel free to ask!