Detail Task View
When you tap a task, you'll go into detail task view. On top, you'll see title of your task. You can tap there and edit if you want to. Also you can tap the circle to check/uncheck. Below the title, you'll see due date, icons and list name. You can swipe to see 'Quick Actions' on left and 'Subtasks and Notes' on right. You can tap arrow on bottom right to move between tasks on the list.
Start dates, Due dates and Duration
If you have a due date, you also have start date and duration. Normally it's setup to be same. If you change your start date, duration will be shown accordingly. Tasks that aren't due yet but if start date has past, it'll be shown on the date list.
Quick Actions include quickly editing due dates, changing lists, priorities, adding/removing/switching tags, duplicates and etc. Quick Actions can also be used on 'Bulk Actions'. Pro users can add unlimited Quick Actions inside settings.
On GoodTask 3, subtasks are saved in notes field of Reminders data. It's shown on json form in Reminders app (Also 'Repeat after completion' and manual sort numbers are shown). You can add subtasks and check it off by tapping circle for each subtask. These are shown on tasks list by (completed subtasks/total subtasks).