No worries. A couple other things:

• /c/community@instance and @community@instance will work for communities.
• /u/user@instance is instance agnostic but will not generate a mention to the user's inbox.
• [linktext](http://instance/u/user) will link to the user's instance only, but it sends a mention to them. You can also do this by starting to type @user@instance and selecting the user from the dropdown box (on the website, not necessarily in apps).
• The different apps for lemmy don't necessarily always work correctly in line with the website, but they'll catch up.
• Each post is actually primarily hosted at that user's instance, then federated from there. So if you're a lemmy.world user and post in a lemmy.ml community, your post will actually post at lemmy.world first and then federate. It isn't necessarily hosted in the community's instance. The same is true for every comment - hence why you won't see comments from users in an instance yours isn't federated with, even if they are federated with the post's instance and can comment in the thread.
• You can't really do anything with posts or comments to jump across instances, as each instance assigns a different number to the same content. Hopefully one day they'll change it so the url is more like http://instance/post/123456@hostinstance in everywhere but the federated host instance. But it's all still in early development.