Select the task you would like to create subtasks for. Click the details arrow on the task.

Scroll down in the task details to "Add subtask". Click "Add subtask".

Subtasks can be assigned to a different person than the original task. 

The same information that can be added to tasks can be added to subtasks by clicking the details arrow.

Subtasks can have their own subtasks but it isn't suggested because it makes seeing the subtasks challenging for teammates.

Things to Know about Subtasks: 
subtasks won't appear on the timeline view of your project
subtasks custom field values won't be included in project's

 Dashboard
subtasks won't be included in measuring workload
subtasks will appear in your MyTasks when assigned to you
the subtasks assigned to you will appear in the Calendar view of your 

MyTasks
subtasks are included in Advanced Search results 
subtasks are included in Universal Reporting

When and How to Create Subtasks