One of the most frustrating things about Linux, and all other flavours of Unix, is the permissions system. It’s not super complicated, but it can get involved, and it helps if you have a clear idea of what you’re dealing with. I don’t use it that often, so every time I need it, I have to dig into it again, and relearn much of it. Thankfully, there are many tutorials for this sort of thing on the web. Unfortunately, the quality of these tutes is highly variable.
I recently had occasion to dive into the chmod command again yesterday, and was lucky enough to find a very good tutorial, complete with examples and exercises. If you need this stuff, it’s good to have it on hand.