Once your child has mastered addition, subtraction will be easy to teach. Using the same tools you used in the previous lesson, start by doing a review of the skills you have developed so far.
When you are sure he has mastered addition, do one last drill. Place three toys in one circle of yarn, and two toys in the other. Have him "write" out the problem, just as you always have. Then ask him to check his answer by placing the set of two in the same circle of yarn as the set of three. Start by having him count how many toys are in the set of three. Then, as he moves the toys out of the set of two, have him continue counting. In other words, he won't start over with one and two. He will continue the count he started in the other circle. The toys in the set of two will become numbers four and five. Have him recount the toys to be sure there really are five toys in the new set.
If he placed the previous addition problem under the sets, then start a new problem above the sets, leaving the addition problem in place. Put a new five over the set. Now ask him to take the toys back out and return them to their old circle. Be sure that in the previous step, you had him move them the correct direction. When he moves them back, the five should be on the left. "How many toys did we take away from the set?" Let him count or tell you. Then say, "We started with five toys." (Point to the new number five.) Then we took away two. (Add a two over the set of two.) "We didn't add, so we can't use the plus sign. We need a new sign. This is a minus sign." (Show him the minus sign and have him repeat the name. Let him trace the sign to help him remember.) Put the minus sign in it's place between the two numbers. "We use this sign whenever we take things away. Taking away is called subtraction. (Practice this word.)
"We had five toys, and we took two away. These numbers tell us that. They say, "Five minus two. Remember, minus is the sign that tells us we took things away." If your child seems a little confused by the set of two, just move it out of the way, saying, "We took these away, so we don't need them. The number two will remind us how many we took away."
"How many do we have left in our set?" Count them out."We have three left." Put down an equal sign. "So, that means that five minus two equals three. If you have five, and you take two away, you have three left. Five minus two equals three."
Next, show the addition problem again, and then repeat the subtraction problem, but this time, take the set of three away. The problems of 2 plus 3 equals 5, 3 plus 2 equals 5, 5 minus 3 equals 2, and 5 minus 2 equals three are called families. They are a set of related problems. Whenever you work on subtraction, practice with related sets. This will help them discover how addition and subtraction are related.
Even if they seem to get it right away, continue working on this and the related sets for a long time. (Work a few minutes every day for as many weeks as you need.) You want this to become natural, and second nature. Public schools cover so many math subjects in one year that the children forget everything. This is why schools re-teach every subject every year. If you just drill in a few subjects, your children will learn the subjects perfectly, and will need little review next year.
If your child can write, you can start teaching him to write out math problems, or at least write the answers to problems you have written out. Try writing the problem on a wipe-off board, and letting your child answer there. He should be allowed to use the toys for a long time before you begin memorization.