Grammar often seems to be a string of petty rules that are and should be easily forgotten.
Much of memory works by either repetition or structure. We tend to forget just about everything else we encounter in a day, and for good reason, but we tend to hold onto information or strategies that we repeat or that fit with the frameworks we have already developed.
So learning new methods of teaching calculus is easy for someone who already knows how to teach calculus because the new strategies can be related to the strategies that have already been developed through years of teaching. And learning new information about Emperor Justinian easy for someone who already knows quite a bit about the Byzantine Empire. But in both cases, these strategies and this information are difficult to hold onto if the person has not already developed the cognitive apparatus for storing and sorting the new material.
The challenge with teaching grammar is finding a way to make the material stick to memory in a meaningful way. A math teacher is going to have a pretty easy time holding onto any strategies for teaching calculus, even if those strategies are different from the strategies she is currently using. That history teacher would also have little trouble holding onto any information that might just be coming out regarding an archaeological study recently completed on Justinian’s tomb. But a student of English grammar has no pre-existing way to file all these new strategies and information he or she is receiving in a grammar class.
This leaves us with a need to repeat material a good deal, but it also means we need to develop connections to cognitive strategies the students already have but also are developing.