Am I missing something? Where is the actual lesson? There are a handful of practices but no actual teaching of new words, other than a few pictures to click on.
The grammar is explained in Duolingo's web version in the "Tips and notes" on the bottom of the lessons page of the skills, which introduce new grammar.
Workaround for App users
The grammar "Tips and notes" are available in the desktop mode of the browser of your phone/tablet.
In the Firefox browser you can select the "Desktop mode" via the three dots in the upper right corner of the screen.
(Note: This may be not working on iOS phones with a small screen)
More grammar information in Duolingo for:
Learning tips, Grammar and vocabulary guides and more