Project tasks are organized in a tree hierarchy. The initial structure is usually created by project administrators; the lower level tasks are then expanded to a more detailed task structure by the task administrators.
Process and project administrators can create tasks in project at any level. Project members can create subtasks to tasks for which they have the task administration permission (see How do I manage single user permissions?).
On the Tasks page, click +TASK button to add a task below the task that is currently selected.
If you do not see the button, it means you do not have a permission to create a task at the project level. Navigate to the task you have the task administration permission for and click the “+” button on the task to add a subtask: