How to Create a Task & Subtask in MS Outlook
You can easily create new tasks using the New Tasks button in the New group of the Home ribbon in Microsoft Outlook 2013's Tasks pane. While Outlook does not provide any native sub-task features, you can apply a workaround using individual task attachments within other tasks. You can create new tasks for each item you wanted to include as a sub-task, and then use the Outlook Item command on the Insert ribbon to attach shortcuts to your other tasks -- effectively using them as sub-tasks.
Create a Task
Click "Tasks" from the bottom navigation bar in Outlook. Select your task list from the folders in the left sidebar. Click the "New Task" button in the New group of the Home ribbon. Enter a subject into the "Subject" field. Fill in any additional information your task requires, such as a due date, priority or category. Add any details for your task into the large box at the bottom of your task. Click the "Save & Close" button.
Attach Task Shortcuts to a Task
Open or create a task for which you want other tasks attached. Click the "Insert" tab and then select the "Outlook Item" button in the Include group. Select the Tasks folder containing the tasks you want to attach in the box under Look In. For example, select "Tasks" under your Exchange account. Select "Shortcut" in the Insert As group. Select the task or tasks you want to attach in the box under Items, holding "Ctrl" to select multiple items, and then click "OK." Click "Save & Close" to exit. Double-click the task's attachments to open the related task.