Adding SubTasks in iPhone
Is there a way to add a subtask to an existing task on the iphone? I see how to mark a task a sub task but not how to associate it with another task so it becomes a sub task of a parent task.
Also, how and where do I see completed tasks on the iphone?
To change the indent level of an existing task to make it a sub-task, please tap on it, go to "Edit" and one of the menu elements (the one at the bottom) will allow you to select an indent level. Pick one, save the task and it will be a sub-task. You can then drag & drop it (long-press and hold the task) to move it below any task you would like to become the parent task of it.
Unfortunately, it's currently not possible to view completed tasks on the iOS app, but we'll consider adding such option to our mobile apps and you're welcome to vote for this request in our Votebox: https://todoist.com/Vote_iOS/showProposal/7/