One of the constant complaints my colleagues and I have about the students is that they do not remember the prerequisite material. I teach math and math is very cumulative. For example, before a student can study quadratic equations, they should have a complete understanding of linear equations.
Sometimes students ask me if calculus is difficult, and oddly enough, it isn’t. There is one concept that is difficult and it comes early: limits. Once that is understood, calculus is very logical and surprisingly easy. However, most calculus classes use a great deal of algebra and trigonometry. If a student doesn’t have a firm grasp of those subjects, he will not understand calculus. This is a repeat of what happens earlier in students’ careers. If they have difficulty adding, multiplication is very hard to understand.
I teach Intermediate Algebra every semester. Many of my students cannot solve simple linear equations which they should have learned, not in the prerequisite, but in the prerequisite to the prerequisite. It is hard to teach students who cannot find the common denominator for simple fractions how to find the common denominator for rational expressions. The concepts are the same. I should not have to teach grade school math.