Long ago, a friend told me that when she was in first grade, she had a lot of pennies. She knew how to add but not subtract so after she spent some, she would have to count them again to know how many were left.
I have finally reached the last lesson (Lesson 12) in my beginning Chinese textbook, which I have been using (fitfully) for more than a year. Later lessons build on earlier lessons. When I didn’t know a word in a later lesson, I scanned the new-word lists from earlier lessons to find it. I have just discovered there is a word index.