What are the dependencies between a subtask and previous task?

• You cannot make changes to a subtask if it has a previous task. You cannot change its status to ‘Completed’ or modify its start time. 
• During the creation of a subtask, the start date and time of the subtask is set to the due date and time of its previous task.
• If an existing subtask with no start date and time is edited and linked to a previous task, then the start date and time is updated with the previous task's due date time (if specified). 
• If an existing subtask whose start date and time is specified is edited and linked to a previous task with no start date and time, then the start date and time of the subtask is not overwritten by the value specified in its previous task.
• You can mark a subtask as completed only after completing its previous task.