It is actually a great thing that Deutsch Akademie offers free Grammar online course. I have tried it and found it very helpful. I do exercises of all kinds, because I want to improve my German in every aspect although I have found the Partizip Perfect exercises to be the most helpful at least for me, because that is what I had more difficulties with- remembering the exact form- since sometimes they end on -t, other times with  -en, and sometimes more letters change, but the more I practice the more I notice that I remember them, and sometimes I don’t even have to think about it- comes to my mind right away.