When learning foreign language vocabulary, repeated practice is essential for success; as words get established in the long-term memory, learners can move on and focus on new skills.
Two effects are at play in this process: the spacing effect, the finding that short practices spaced out over time is better for learning than cramming; and its related finding, known as the lag effect, which states that learners improve if they gradually increase the spacing between practices.
These ideas go back to 1885, when German psychologist Hermann Ebbinghaus pioneered the concept of the forgetting curve. He tested his ability to remember a string of words over different periods of time and found a consistent pattern to the decline of his ability to recall these words over time. Immediately after the learning experience, his recall was 100 percent, but memory dropped steeply the first few days. Further, he found that the memory loss was exponential, meaning it increased by the square of the previous number until finally flattening out at around 30 days post-learning.
According to Ebbinghaus’ findings, the way to counter the forgetting curve (i.e. learners are more successful) when they plan short practice sessions and gradually increase the amount of time between each session.
 ‘Repeating list items leads to better recall when the repetitions are separated by several unique item than when they are presented successively; the spacing effect refers to improved recall for spaced versus successive repetition (lag > 0 vs. lag = 0); the lag effect refers to improved recall for long lags versus short lags.’
– Kahana. M.J., Howard. M.W. (2005) Spacing and lag effects in free recall of pure lists Psychonomic Bulletin & Review, 12 (1), p. 159-164