Lunatask implements subtasks using checklists in task descriptions. This way, you can give any structure to your subtasks, including nesting them.
To create a subtask, click the checklist icon in the formatting toolbar. Alternatively, type
[ ] and a checklist will be created.
To indent a line, click the corresponding icon in the formatting toolbar, or use
Alt+Arrow Right keyboard shortcut to indent and
Alt+Arrow Left to unindent the line.
When creating subtasks for recurring tasks, completing or skipping each occurrence will uncheck all subtasks, so each occurrence gets a fresh start.