Hi @Angelika_Seaman, welcome to the Asana Forum! When you delete a task, all subtasks that are part of another project remain available and are not deleted. This is currently expected behaviour, see more details in this thread: Delete task - subtasks don't get deleted - #2 by Phil_Seeman
If you would like to delete all remaining subtasks, please see the steps here: Bulk delete subtasks and sub-subtasks - #2 by lpb
I hope this helps!