When you export and import… all the subtasks move up to the task level. We have complex project with many subtasks… THIS IS NOT HELPFUL!!!
Please fix!
Welcome to the Asana Community Forum @Candice_Hargett 
Have a look here this might help: Identifying Subtasks in export - #2 by Andrea_Mayer
Not helpful… it should automatically add a column to the export and map the subtasks up upload. We could never get it to work.
I see, then do not forget to upvote on the top.
Also if you want the same functionality for the import here is the relevant feedback request thread for it: Ability to import subtasks with drop-down fields via CSV