I mistakenly clicked on 'I know this quite well' on katakana ka and now I cannot get the flashcard with the ketter and its details back... It is like.. just like it's gone forever! I wanna view the mnemonic of it but it's gone. Please tell me how to get it back...
Tap the name of the schedule (for example, Hiragana) to get to the schedule's summary. You can see all terms separated into groups (studied, not studied, hidden, etc.). For terms you mark as quite well, they are probably in the level 6-8 range, which should make them easy to find.
Once you've found it, if you want to reset the level and learn it normally, one of the view options in the term list is "Tinker", which will allow you to change the mastery to 0 and unknown. After that, it should be re-introduced like you first saw it when you learn new terms in that schedule.
