Message # 1 | 9:39 PM

I´m building a forum using your services. I know how I can sort the forums into categories. I wantes to know how can I make a sub-category, for example:

Industrial Engineerings (category)--> first year (sub-category) --> Fisics (forum) --> topics

I already know how to make the first and last part, but i dont know how to make the middle part of the example above.