I was a substitute teacher for a time. Because retaining teachers is such a problem these days I ended up with a few long-term assignments. As such I have some experience writing lesson plans. On my single day assignments I have also had the opportunity to see many different teaching styles.
There have been many viral homework assignments renowned for how difficult they are to understand. Allow me to shed some light.
Common Core is a set of standards that must be met. For instance: students at a certain grade need to understand the concept of the place system (100s, 10s, 1s etc.). However the requirements are written so convoluted that they are nearly incomprehensible. (See whole site.)
Decompose numbers less than or equal to 10 into pairs in more than one way, e.g., by using objects or drawings, and record each decomposition by a drawing or equation (e.g., 5 = 2 + 3 and 5 = 4 + 1).CCSS.Math.Content.K.OA.A.4
For any number from 1 to 9, find the number that makes 10 when added to the given number, e.g., by using objects or drawings, and record the answer with a drawing or equation.
Common Core also limits the teaching techniques allowed. The shortcuts that many learned are banned. Even the way that most people do math is considered a shortcut. I respect that common core wants students to understand the concepts behind the math. When I was in school, we did. Once we had the concepts down, then we learned the quick and dirty shortcuts. As far as I know that is true of most of us. I know we may not all be experts at math but if I asked where the 10s place is, most grown ups get that. I learned with blocks how borrowing for subtraction worked. The problem seems to be that common core takes away things, without giving. There are ways that teachers are not supposed to teach, but they have not been taught any alternatives, so they have to come up with their own. Common Core is not a curriculum (how to teach), but only a set of requirements (what to teach).
There is such a focus on making sure that kids understand the how, that they get over-taught it. I understand counting up and down on a number line or your fingers. I even get skipping down that number line in larger units. I understand adding 4 and 6 fist in a big line of numbers to reach ten then add the harder ones. Some teachers struggle to interpret the question and end up teaching that in order to add 4 and 8 you have to reach 10 first. (4+?=10…4+6=10…8-6=?…8-6=2…so 4+6=10+2 leftover =12.
I understand that learning the concept is more important that getting the right answer all the time, but once the concept it there, learning to write it in a simple way is hardly a shortcut. If taught correctly, it is the how.
The other BIG problem with common core is the requirement of ever younger kids learning ever more complicated topics. I had a kindergartner in tears because he was not developmentally ready to learn subtraction. When I was in kindergarten we were still being taught to count. This leads to a frustration with learning, and a lack of self esteem that most children will never recover from.