Microsoft has updated its To-Do task managing app today with to add support for subtasks, which has been the top-requested feature from users. Microsoft is calling this new feature “Steps,” and we’re seeing it today on all platforms except the Android app
When you create a new task in To-Do, you’ll now see a new “Add Step” button below the title. You can create many subtasks if you want to, though It’s worth noting that marking all the subtasks as completed won’t mark the main task as such, and vice versa (as seen below.)
Microsoft is still working on many more user-requested features for To-Do such as integration with Cortana and the ability to share lists between users. If you're interested, you can track Microsoft's progress on these new additions and help the company prioritize new features on the dedicated UserVoice website.