I consulted a grammar book again when I was trying to find an answer to a question this morning - it's been a long time since I last read the book. When I was looking through its table of contents, I found I needed to read some of its items again because I wasn't so sure if I really understood them, or rather some of the entries were so unfamiliar to me now even though I had read them more than once.
Yes, people forget things, and repetition seems to be one of the most effective ways to learn. So