Years ago, when I was studying Japanese, I had a collection of over 10,000 flashcards in my SRS. That’s a huge collection, and reviews were pretty long. The idea was that having an SRS with everything I ever learned in it would ensure that I would never forget more than about 10% (that was the forgetting index the SRS was set at). However, during a long period of time of learning no new material I noticed that my reviews were gradually getting longer and harder. At the end of my learning period they were less than 1 hour. After several months of no new cards, they were approaching 1.5 hours. I ended up deleting the entire collection in a fit of rage, and it felt great. But I wondered why that happened. I have several theories, and I think more than one thing was going on there, but the one that pertains to this post is the addition of new material actually improves recall of old material. This is one of those language learning things that doesn’t sound very obvious. But after I came up with that idea, I noticed several things that seemed to confirm it. I spend a lot of time maintaining languages, which by definition is only using it, and not trying to progress in it. This is several years in the past, when I just used certain skills and noticed a slight drop in my level over months of maintenance. But whenever I looked up a words or grammar I would notice a slight improvement in my state of mind. During the maintenance I was actually losing ground and forgetting stuff, and the new material was replacing it. But it was more than that – I got the feeling that I was actually learning instead of just maintaining. It was as if the new material was churning up all the old material and keeping it closer to the surface. So I was starting to believe in this, since I seemed to be getting empirical evidence. A couple of years ago I read an article covering an experiment that seemed to confirm adding new material improves recall of old material, and memory in general. It was a simple flashcard experiment. Unfortunately I can’t find a link to it, but it was pretty popular on the language forums for a few months after they reported it. I was really happy to read this; I felt sort of vindicated. Discussions on the forums showed that many people already knew this, and were using it to their advantage. My conclusions are that whatever you are doing, try to learn a little new material too. It doesn’t have to be a lot in my experience. Even something as simple as looking up a word, memorizing a phrase, checking the grammar of something you heard on TV, etc, can improve the effectiveness of your learning. The main thing is, don’t get into a long stretch where you only review old material. New material will not only improve your recall, but it will liven up your study.