So I know a bit about moderators but I always wonder: why do moderators keep there discussion locked? If anyone knew this it would be a big help to comment down below.
Discussions by moderators are not necessarily locked. Rather, posts that are not discussions are, even if not made by moderators. This means that a post may be locked because its content is not intended to open a discussion, like for example, posts created to deliver certain information. Here is an example (click on this gray text) about a post that is locked. It is made by a normal user to provide a text formatting guide when using duolingo forum.
Mod here. It is exactly as stated by Archnje. The Discussions are open to the public what is locked is the ability to add comments. This is done simply to avoid cluttering the site. Sometimes another link will be used for discussions. In any case, you can always ask for clarifications or make other comments directly through the Discussions.
BTW the link that Archnje uses in the example is one of my favorites. You should check it out.