Give students ample opportunities to practice basic addition and subtraction facts. Second-grade students will not understand regrouping if they cannot perform simple functions. Review place values. Children must grasp, that in the number “23” the “2” represents “20.” Give the students double-digit addition and subtraction problems that do not require regrouping, such as “11+11” or “29-27.” By reviewing basic double-digit sums, your second-graders will understand the more complicated process of regrouping.
Explain how regrouping affects addition. Write a double-digit problem in vertical form on the board, for instance “12 +18." Ask students for the answer of the ones place in the problem on the board. For the sample problem, the students should answer, “2+8=10.” Tell the children to write “0” in the ones place. The “1” in “10” is in the tens place, therefore, the students must carry it over on top of the tens place in the equation. Instruct the students to draw a small box around the “1” they carried over to the tens place. The box acts as a reminder to keep the carried-over number from changing the value of the added numbers. Ask the children what “1+1+1” equals. The final answer should be “30.”
Once your second-grade students have mastered addition with regrouping, teach subtraction with regrouping in the same way. Write an equation in vertical form on the white board, such as “30-12." Explain that the traditional method of subtraction does not work because you cannot subtract a bigger number from a smaller one. Cross out the “3” in the top number in the tens place and write “2.” Add a “1” in front of the top number in the ones place to create “10.” Write the answer to “10-2” in the ones place and the answer to “2-1” in the tens place. The final answer is “30-12=18.”
Many students will benefit from using manipulatives to understand and practice regrouping. You can use different colored beads, blocks or place-value strips. With manipulatives, the children can move blocks or beads to the tens place, when they are regrouping.
Complete several equations on the board with students until they understand the concept. Give students their own practice while you monitor their progress. The more repetition they encounter, the better they remember to use regrouping in their computations. Some children will find this new concept challenging. Allow those students extra time, assistance and patience.