Children start with a small group of objects, remove some, and count what remains. Separating is the concrete foundation for the concept of subtraction.
Use objects to show a group, remove some to a separate pile, and count the remaining objects. Act out contexts ("There were 5 cookies, you ate 2, how many left?").
Combining the separated groups back together instead of counting only what remains. Counting the removed items instead of the remaining items.
You have already learned one-to-one correspondence — matching each object to one number when you count. Now you are going to use that skill in a new situation: starting with a group of objects, taking some away, and counting what is left. This is the concrete, hands-on foundation of subtraction, even before you ever write a minus sign.
Imagine you have 5 grapes on your plate. Your friend takes 2. How many are left? The key action is physical: you move 2 grapes to a separate place — the "taken away" pile. Now you look only at what remains on your plate and count: one, two, three. Three grapes are left. The separation into two piles (the removed group and the remaining group) is what makes the thinking clear. You need to keep the groups apart, because you only want to count the "still here" group.
The word story matters a lot here. "There were 5 cookies and you ate 2 — how many are left?" That word *left* is pointing you to the remaining group. Other words that point the same way include "remaining," "still have," and "not eaten." Learning to hear these words and translate them into the right action (remove some, count what stays) is an important part of the lesson.
This separating action is exactly what subtraction means: 5 minus 2 equals 3. For now you do it with objects in your hands. Later you will do it with pictures, then with written numbers. But every time you see 5 − 2, you can picture those grapes — the whole group, the removed group, and the group that stays behind. That mental image is the meaning behind the symbol, and it will stay useful long after you have memorized the number facts.