Hover on a task, grab the "handle", and drag the task from left to right to adjust its sub-task level.
Use the keyboard shortcut CMD+Left/Right Arrow key (for Mac) or CTRL+Left/Right Arrow key. Please note that this does not work in the Today/Next 7 Days view or any filtered views.
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.