Sometimes, a major task has several milestones that make it look as if the task isn't being worked on but is, in reality, being handled. Adding all the sub-tasks under such a complex task brings a much-needed structure and a clearer picture of how the task is progressing.
Adding a sub-task to an existing task
To add a sub-task to a task:
Navigate to Tasks, then open the existing task
Tap the Add subtask button, then input a name for it
Click the contact button, locate a teammate you want to assign the sub-task to, then tap Done.
Tap Update to wrap up the process.