Teach me subtraction

So for starts I would give the example of apples. So If John has 4 apples and he Eats 2 (Subtracts) how many apples does John Have? To start off you would get 4 and count down as many apples John ate. So 4-1 apple is 3 apples, then 3-1 apples is 2 and since John only ate 2 apples He is left with 2 apples since he didn't eat 3 to make 1.

Just count backwards 12-3 = 12, 11, 10, and 9 so 12-3=9