The Lengalia trainer is based on Hermann Ebbinghaus’s findings on the psychology of learning. Ebbinghaus found that you forget the new things you’ve learned over the following days, but that by practising your new skills you improve more and more. Therefore, by repeating vocabulary you will be able to remember it more easily. At the beginning, you should revise what you’ve learned after a short stretch of time, but later you should take longer breaks, returning to it regularly over a long period of time.
Words that you soon forget should be revised very often; that way, once you’ve learned them you are unlikely to forget them.