In case you haven't found them yet: When you're learning on the desktop version, you can find them by clicking one of the lessons and then "tips". I'm not sure if you have them on the app as well, I don't, at least for Norwegian, so I'd recommend using the desktop version. If that's not always possible for you, do that for every new lesson one time to take your own notes for the grammar stuff, it's really helpful :)
In what instances would you use 'den'? Would an animal be a 'den'? Is there a substitute that one could use as to not call an animal an 'it'? Or would that just be han and hun? In English, 'they' can be substituted for 'it' which makes it sound less like an object and more like a subject, even though it is grammatically dubious. Sorry if this question sounds confusing...