Children physically combine two small groups of objects and count the total. Combining is the concrete foundation for the concept of addition.
Use objects (counters, blocks) to show two groups, then push them together and recount. Say the story: "I have 2 blocks and 3 blocks. Now I have..." Repeat with different numbers.
Recounting the first group when combining (not finding the difference, just the sum). Losing track of groups when combining.
You already know how to count a group of objects by pointing to each one and saying its number — that is one-to-one correspondence, and it is the skill that makes combining work. When we combine two groups, we are asking: if I count all the objects together, what number do I end up at? The action of physically pushing two groups into one pile is the most important first step.
Here is how it works: start with 2 red blocks and 3 blue blocks. Push them all together into one pile. Now count the whole pile: 1, 2, 3, 4, 5. The answer is 5. The colors do not matter once the groups are combined — we are counting the total. Doing this with your hands, moving each block as you count it, helps you keep track and not skip any.
The story matters too. We say: "I had 2 blocks. I got 3 more blocks. Now I have 5 blocks." This is the same action told as a sentence, and saying it out loud connects what your hands are doing with what numbers mean. Later you will learn to write this as 2 + 3 = 5, but for now the story and the action are the thing.
The most common mistake is starting over when you count the second group. If you have 2 blocks and you push 3 more in, do not start counting from 1 again — count all of them together from the start. Keeping both groups in front of you before you push them together helps you remember that you have two separate amounts that are becoming one.