We often interchangeably use the two concepts: memory and learning. The two concepts are intimately intertwined, but they are of fundamental difference. Learning focuses on the acquisition of knowledge or skill, whereas memory refers to our capacity to later recall it. Learning is the initial impact of an experience and memory is its subsequent effect.
Hermann Ebbinghaus is a German scholar who first conducted the very first experimental study of memory. He created and memorized nonsense syllables (meaningless material he would never have seen before) and read aloud the list, then immediately tried to repeat the list in the correct order until he reached a single perfect recitation. One major finding from this experiment was that practice was proven to be an important determinant governing memory. Memory depended not just on the frequency of practice, but also on how this practice was distributed over time. Recall proved to be better when the practice was distributed over multiple days. Ebbinghaus concluded that with sufficient practice, the material would effectively be remembered permanently, with no loss over time.
Q: Why do we forget things we used to memorize?
As the amount of practice on a list increased, the amount of forgetting fell off sharply. Forgetting was very rapid over the first hour but then declined only very gradually over subsequent days. In addition, memory can be retained for longer terms when there is an opportunity to review or refresh their memories at periodic intervals.
[Reference] Lieberman, D. A. (2012). Human learning and memory. Cambridge University Press.