If I start a thread or make a comment on someone else’s thread, how can I delete what I’ve written?
You can't delete a discussion. The idea is it wouldn't be right for the person opening a discussion to be able to delete other people's posts just because they decided they don't want to talk about that topic anymore.
In regards to limitation of deleting your own posts, that's more to help prevent disruption of flow and confusion. But you can still technically remove the contents of your own posts. A mod may then delete it once they see it.