One of the biggest struggles in learning anything is reflecting on your own progress. The inability to see something build over time deflates motivation and hold's us back.
Memory Lane solves this problem through natural language. A user can see their progress written in plain text. Based on knowledge of when an individual learned a word, how well they know that word, and knowledge of all the words they know in detail, we are able to show the user sentences they understood a month ago, a week ago, today, and even what they should expect to understand in the future.
The earlier sentences will be simpler both in vocabulary, grammar, and length. As someone progressing through a language, these variations will very clearly highlight actual progress, much more than a graph could hope to.
An additional use for this is example sentences in a dictionary. Looking up a word in the present will produce example sentences at the level of the user, and include personalized content based on their interests. After another week of study, looking at the same word will produce new example sentences at the user's level.
These both are teaching important words that the user need to study anyways, and providing insight that the user couldn't easily see otherwise.