There are 2 ways to create subtasks. From a task list, press and
hold your finger on a task and choose the option to add a subtask to that task.
Or, you can create a subtask from the task viewer (but not the task
editor). When viewing the details of a task, tap on the button at
the bottom of the screen to add a task and the app will give you the
option of creating a subtask from the current task.
If you don't see these options in the app, then you need to enable
subtasks. Go to the settings area and tap on "fields/functions
used", and make sure the "subtasks" option is checked.