This rule also works with the genitive (possessive). As in degeberg's example, you could replace "den" with "min" (my), "din" (your (talking to one person)), "jeres" (your (talking to more than one person)), "Sørens" (Søren's)
The lesson notes aren't available on the mobile apps, they are only available on the website. When you click on a skill, some of them have grammar notes below the lessons. These can also be accessed in the lesson in the top left corner.
One of the moderators in the general forum claims that the majority of people are accessing DL from the mobile app only. If this is true it is a shame they don't make a way to access the notes by app as well.