Creating folders within folders (subfolders) isn't supported just yet. You can help us by filling out this feature request form and let us know how you'd like to see subfolders implemented.
With Class Folders, you can keep relevant assignments together in one place! You can even choose colours for them so you can differentiate them at a glance.
Here's how you can create them:
Choose the class you want to create folders for
Tap the wrench icon
Tap New Folder
Type in your folder name and choose a folder colour
Tap Save
Once created, it'll take you directly into the folder where you can start creating your assignments!
If you already have assignments setup, you can move multiple assignments quickly. This guide can show you how.
Rearranging Folders

Tap the wrench icon
Tap Manage Lists
Tap and hold the reorder handle (the three bars) next to the folder you want to move
Drag and drop the folder in the preferred order
Tap Done
Folder Settings

Select the folder
Tap the wrench icon
Tap Folder Settings
Edit the title or colour
Tap Save
Subfolders in Showbie isn't supported just yet. You can help us by filling out this feature request form and let us know how you'd like to see this feature implemented.