We can add a new sub task to a given task by clicking the 3 dots … on right hand side while on the task page and then clicking create sub-task and in task we can fill as the fields like
Normally sub-tasks are assigned to different team members, but they can be assigned to same team members as well.
Please note that once a sub-task has been added to a task new sub-tasks can be added from sub-task section as well by pressing the + sign.
Also note that once the status of each every subtask is marked as done only then we can mark the status of a task as done.