Sub-tasks are definitely one of the most requested features to add in AceProject. We are still unsure as to whether they will be added someday or not. There are technical reasons, but also usability reasons. Actually, sub-tasks wouldn't be used by everyone, and most users who would, don't need all the sub-tasking properties available in products like Microsoft Project. Our biggest challenge is to keep sub-tasks as simple as possible, and usable by anyone.
That being said, there's a workaround for sub-tasks in AceProject that works for many users. It simulates sub-tasks, although it doesn't replace them per se.
All you need to do is use integer task numbers for tasks and decimal task numbers for sub-tasks. If necessary, you may set dependencies between specific tasks.
Here's an example:
We are aware that this is not ideal, but it works pretty well for basic sub-tasking.