Learning German grammar on the internet can be fun! Here are 3 websites that make learning about cases, gender, prepositions etc. a bit more enjoyable and easy :) Again, all of this is free!!
Lots of grammar topics and exercises that are all related to fairy tales..learning German has never been that enchanting! ;)
If I want to share quick and easy explanations about Grammar topics, i always use Lingolia, it’s very structured and offers great exercises. Besides grammar, you can also learn about vocabulary and regional/cultural Information there.