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It depends on what you want to train (grammar, vocabulary...) One of my favorites is the free trainer "King Of German Article".

https://play.google.com/store/apps/details?id=com.devtrigger.article.free.german&hl=en

It helps you to train nouns and the right gender with it. You can also train the different cases Nominativ, Akkusativ, Dativ. As for making grammatically right sentences you must know the right gender, this app helped me to improve my German a lot.

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