Cutting and pasting

Hi, I would like to cut and paste a task - which has numerous subtasks - from one MS project file to another. So I collapse the entire task - right click on the 'row number' of the task to copy the task & subtasks, then right click and paste to insert this task & substasks at the top of the other MS Project file. This seems to work OK but (just checking the task row) the pasted data does not keep the start date constraints. How can I copy the task & subtasks from one sheet to another and keep the date constaints please?