Yet when it comes back around with word problems, data analysis, elapsed time, and division, suddenly they forget everything they have learned. They forget that you can't subtract 5 from 2, and they will tell you that 42-25 =23. They will do this because they will forget all about regrouping. They will even make silly mistakes like forgetting to take a ten when they add ten ones when regrouping.
This is the reason why I spend so much time on subtraction and building it's foundation. Students must thoroughly understand it and it's process. I also spend a lot of time teaching them different ways to subtract. Some ways require them to remember very few steps. Some involve counting and not remembering facts. And other methods eliminate the need to regroup at all. Whatever way they choose to subtract, they will have a thorough understanding of subtraction when we finish this unit. An understanding that will, hopefully, stick with them throughout all of our math units!