To the right of any task's date box are two arrows. Clicking on the right arrow indents the task and makes it a sub-task of the task above it. The task above will now have a triangle to the left of its name. You can use the triangle to hide or view any sub-tasks below it.
Also, any time you choose "Add task below," the new task is set at the same level, so if you add a task below a sub task, it will also be a sub task of the task at the top.
I tried to add sub task in iOS app, and didn't notice it's a sub task, the indentation of sub task is slightly unnoticeable.