There are no built-in subtasks in Hygger. But you can create them in 2 ways:
Using Task Links
Organizing subtasks with Task Links
Open a task
Click Add.. > Task Link
In Link Type field choose Is Subtask Of (if you don't have this link type in the list, you can create it on the Company Settings=>General=>Task Links Types=>+ Add New Link Type)
Organizing subtasks with Checklists
Besides usual items, you can also add tasks from various boards to checklist fields. Just click Add a task, start typing task's name and the system will give you corresponded suggestions.