Success in each grade is determined by successfully understanding the concepts in the prior grades. Those concepts are used to teach concepts in later grades. But they can be forgotten for a variety of reasons. One needs to go back and review/relearn them when necessary. 1st grade concepts aren't just for 1st graders. You might be in 4th, 7th, 12th, an adult - if you need to understand the concept, you need to understand the concept. It doesn't matter when it was first introduced. Use the charts below to help remind yourself what one needs to have understood in prior lessons so as to be able to understand current and future lessons.