In my personal opinion, there should be an area to practise grammar. Grammar is just important and it can be difficult to learn the grammar by reading the sentences, especially in German!!
For now, there are grammar introductions in some skills and you can always use Discussions to clear up all the difficulties. Look at the most popular discussions in every skill, they usually contain questions and answers about the grammar. If there are none of these, you can post a topic yourself. And don't forget there is plenty of grammar in the internet. Don't hesitate to search.
I think the team will eventually improve the grammar sections, but Rome was not built in a day. Duolingo is developing :-)