Tasks can be arranged in a hierarchy, with tasks containing one or more other tasks within a project.
The Task section of the Task details page contains a related tasks table. This shows the parent task and any child tasks. It also shows Dependencies which are described in the next section.
You can add sub tasks to this task using the New sub task option, which is available from the context menu or just below the related tasks section.
The new sub task option works just like New task, and you can use the task wizard by selecting the Task wizard button on the new task page.
You can use Move and Copy to modify the hierarchy. See Plan view for details.