Please note that sub-tasks can only be created in a project view and aren’t possible in the Today/Next 7 Days view or any filtered views.
Web, OS X, Windows (Legacy)
Hover over a task, click and hold the three-line icon next to the task's name, and drag it from left to right to adjust its sub-task level.
You can also use the keyboard shortcut Cmd+Left/Right Arrow key (for Mac) or Ctrl+Left/Right Arrow key (for Windows).
Creating a sub-task while adding a task
Creating a sub-task using an existing task. Please note that this only works within the project view and not in the Today, Next 7 Days, or any filtered views.