Regrouping

Once a student has the concept of 10 to 1 size relationships in base 10, then grouping in addition and ungrouping in subtraction follow.

For example, if you have 4 boxes of candies and need to give away 2 boxes, 7 bars and 2 single candies, you need to open a box to get enough bars and open a bar to get enough single candies to give away.

Having something concrete to relate to gives the student more confidence and a mechanism for connecting familiar experiences to more abstract concepts.