On each task you can add unlimited sub-tasks from sub-task section.
Just open task and scroll to subtask section.
Click “Add subtask” to create new subtask.
You can check each subtask to mark them as finished.
Edit, delete subtasks
You can easily edit or delete subtasks by clicking subtask text. Then you will be able to edit / delete subtask.
Subtasks also can be reordered. Just drag and drop subtasks to desired position to reorder them.