4) What is sprint?

Sprint: It is also known as iteration. It is period of time or deadline where a certain amount of work has to be completed and be ready for review. Sprint can only be applied to scrum board. It cannot be apply to kanban projects
b) Subtask cannot be moved independently of their parents.
c) An issue can only be assigned to one active or future sprints. it means you cannot assigned active or future sprints both at the same time.
d) You can create sprint for iteration or multiple sprints if you want to plan several iterations ahead.