We think for German, new syntax cards would work. We'd do the following:
With an example sentence like this:
Er gibt einem Mädchen einen Kuss.
He gives a girl a kiss.
Blanked out sentence: Er gibt __ einen Kuss. [picture of girl]
Back(Word): einem Mädchen
Root Word: das Mädchen
Always Back: einem –> Dativ (masc./neutral)!
And do the root word tests.
Then we’d get 4 cards, two asking you about the declined form (einem Mädchen), and two asking you about “das Mädchen”, all with a note on the back reminding you that “einem” is Dative.
In addition, we'd set up some mnemonics for the cases and make some flashcards to remind you about them. In tinkering around with various imagery, we've found that you might like using gestural mnemonics for noun cases.
[STANDING ON OBJECT] = nominative
[POINTING AT OBJECT] = accusative
[HEADBUTTING OBJECT] = dative
[GRABBING/CRUSHING OBJECT] = genitive
Then if we see a sentence like:
Er gibt __ einen Kuss.
And we get the case wrong, then you can re-imagine the story in the sentence with the guy headbutting a girl, and then kissing her, to help recall the proper case we’re after.
Same thing if we can’t remember which case “einem Mädchen” is in when we see it on the comprehension card. If we see that phrase and we can’t remember what case it’s in, we’ll think about the imagery of people headbutting girls.
[Apologies for the violence, it’s just how we find one stores things better!]