Intervals does not feature traditional subtasks but milestones can be used in a very similar fashion. With milestones tasks become a child (or subtask) of its milestone and milestones do have quasi dependencies since you can group tasks together with one milestone and use the milestone to move all of the tasks that are associated with it. This video overview of milestones might be worth a look http://www.myintervals.com/video/#Milestones if you haven't experimented with milestones much.