Interactive German Grammar
A clever strategy to acquire/learn various aspects of German Grammar has been developed by the Goethe Institut. You do need a login/pw to reach the site but can register at Goethe.de for a free ID and password.
The approach requires an intermediate facility in German to understand how the program works and how to use it. Thus it is not ideal for rank beginners, but ideal for those who have acquired some german and need or want to know the grammar rules for the formulation of some German structures in order to improve the corectness of their speaking and writing.
The approach is to consider some examples and from these deduce the rules and then check those rules using several examples. An excellent and elegant interactive approach in which the student discovers the grammatical rules rather than memorizing a list of them, which are then often quickly forgotten.
There may be other such modules but i have been unable to find the links.There must be a general index to all such modules and am awaiting an answer to that question from the Goethe-Institut.
A Deutsch für Dich section provides exercises for the user on various structures at various levels of fluency. https://www.goethe.de/prj/dfd/en/home.cfm
The link to a general online site: kostenlos Deutsch lernen is: https://www.goethe.de/de/spr/ueb.html
A systematic review of German grammar can be found at the Dartmouth,edu site. It is excellent at providing both many examples of use as well as a description of each subtopic as used in both German and as background in English.
Meines erschtens ist diese Internetseite komplett, gut verständlich und durchaus nützbar für Duolingo-Studenten, die unsicher sind wie man etwas schreibt oder genauer wissen wollen was granümatisch richtig ist. Etwas für jeden!
The site also provides an alternate exposition of Wortstellung (word order) that is more complete and useful that that in the Dartmouth main grammar review. http://www.dartmouth.edu/~deutsch/Grammatik/WordOrder/WordOrder1.html
So if you are interested in word order in German I suggest you look both at the above link to the alternate presentation as well as the several word order links in their main grammar table