(If you want) to find this time,(you must) subtract 7-8 hours before your alarm time.
所以在这里To find this time是虚拟语气;后半段的主语是you不过被省略掉了,subtract是谓语.
(If you want) to find this time,(you must) subtract 7-8 hours before your alarm time.
所以在这里To find this time是虚拟语气;后半段的主语是you不过被省略掉了,subtract是谓语.