Navigate to the parent task where you want to add a subtask.
Click the Add subtask button (or click the subtask icon and then + New task if there are existing subtasks) from the Task view panel.
Type a name for your subtask.
Press Enter on your keyboard.
Your subtask is instantly created and you can click it to open and edit. A link to the subtask's parent task is located immediately above the subtask's title in Task view.
Yes. Learn how to do that here.