I'm not sure about setting the genre from the folder structure, but there is an easy way to do this manually. This method assumes you have
just entered a new group of books, all from one genre/sub-genre. The first two steps can be ignored if you're already displaying the Modified field/column.
- Right click on any column heading and Show Column>Modified.
- Left click on the Modified column's heading to sort descending (most recent to oldest).
- Highlight the books you want to work on.
- Edit metadata in bulk.
- Click in the Add Tags field and enter the genre/sub-genre term you want to use for these books.
- Click the Ok button
Just be careful with the timing. Calibre changes the timestamp value of the Modified field after performing some operations that you wouldn't expect. For instance, if you use Edit Metadata and press the Next or Previous buttons whether you actually change anything or not, the timestamp is still updated. So if you're not careful you could accidentally include, and update, a book that isn't a part of the genre you want to work on based on this new timestamp.
You only have to use this method as many times as you've got books to add that fall into one genre. Of course, if you have a book that is in two genres, you're going to have to do that one completely manually for the 2nd term because your timestamp is going to change when you apply the 1st term.
You can use Window's standard highlighting methods in the book listing if the Modified field cannot be used (for books already added into the database).