Teaching regrouping math problems in 2nd grade is one of those things that your college education classes just can’t prepare you for. No matter how great your lesson plans are, students are still going to make regrouping mistakes. It’s inevitable. But don’t worry, you’re not alone. As teachers, we’re in this together to share our struggles, talk about why regrouping mistakes occur, and brainstorm ways to help our students overcome them and regroup like pros!
That’s why I’m writing this blog post… to break down 13 of the most common regrouping mistakes kids make, why they make them, and how we can help fix them. Consider this your go-to troubleshooting guide for all things regrouping!
Before we dive into this tricky topic, let’s start with a good laugh. I’ve never created a graphic that resonated more with my audience of teachers than this one! Can you relate?

How to Teach Regrouping
Teaching children addition and subtraction with regrouping can be a challenge because it requires more than simply memorizing steps. Regrouping demands a strong understanding of place value and how numbers work.
To truly grasp how to teach regrouping effectively, kids need to understand the why behind the process before mastering the how. When they learn to follow an algorithm without fully comprehending it, they are more likely to forget the steps and make consistent errors. These regrouping mistakes often show up in regrouping math problems when students lack a solid foundation in number concepts (especially place value), making it crucial to prioritize deep understanding over rote procedures.

Why Do Children Make Regrouping Mistakes?
While there are a ton of regrouping mistakes that students can make (which we’ll cover in this blog post), most of them stem from a lack of understanding of key concepts like place value, math facts, and the purpose of regrouping itself. If a child doesn’t fully grasp that a digit in one place represents 10 times the value of the number to its right, they may struggle to recognize why they need to “borrow” or “carry” during a regrouping math problem.
In this blog post, I am mainly going to focus on the standard algorithm. I strongly encourage you to introduce your students to multiple addition and subtraction with regrouping strategies before trying this step.
Additionally, some children rely solely on memorizing steps without understanding the logic behind them. This can lead to confusion when faced with slightly different regrouping math problems. Misunderstandings about which numbers to regroup, where to place the regrouped value, or when/why regrouping is even necessary are common pitfalls that can lead to consistent and frustrating regrouping mistakes.
13 Common Regrouping Mistakes (And How To Fix Them)
Mistake #1: Lack of Place Value Knowledge
The most common regrouping mistake that I see, as well as so many teachers in my online community, is a lack of place value knowledge. Without the basic foundation of place value and understanding that 10 ones is equal to 1 ten or 10 tens is equal to 1 one hundred, regrouping is bound to be a challenge for them and they will struggle to add or subtract by the place value (which is what regrouping is all about!).
When a student struggles with these foundational skills, where should you start?
When they aren’t understanding the standard algorithm, what should you try?
Use this checklist below to help teach regrouping using the CPA approach:
- Regrouping using groupable models such as linking cubes and a place value mat.
- Regrouping using pre-grouped manipulatives such as base ten blocks and a place value mat.
- Regrouping using pictorial models, such as drawing out a set of base ten blocks to solve.
- Regrouping using more of an abstract model, such as solving on an open number line.
- Regrouping with abstract thinking with the standard algorithm.
As you work through this checklist and teach regrouping math problems, a few questions to ask yourself are:
- Can they solve using pictorial models?
- Can they solve using pictorial and written expressions?
- Can they solve using only a written expression?
When you follow a CPA approach when teaching regrouping, it’s easier to spot student misconceptions and determine a course of action that meets them where they’re at. You’ll be able to see if they’re struggling at all levels, even a concrete level, or if they begin to struggle as they conceptualize it with pictures and abstract algorithms.

Mistake #2: Not Lining Numbers Up Correctly by Place Value
When students fail to line numbers up correctly by place value in addition and subtraction with regrouping math problems, this throws them off from the get-go. When the numbers are misaligned in a problem, such as adding a three-digit number to a two-digit number, this often happens because students don’t fully understand the concept of place value. They may see the numbers as a sequence of digits, rather than grouped by ones, tens, and hundreds. For example, when faced with a problem like 345 + 78, they might align the 7 under the 3 instead of under the tens column because they’re focusing on the position of the digit, and the first two digits align.

So what does this tell us as teachers?
This indicates that the student is thinking more about the appearance of the numbers than their meaning.
To address this common regrouping mistake of not aligning numbers correctly by place value, start by expressing the importance of adding the ones place first when you introduce adding and subtracting multi-digit numbers. Start with place value charts and concrete objects to give students a visual guide of how numbers are structured.

My school’s curriculum text book primarily focuses on writing addition and subtraction problems horizontally, and it drives me nuts! Just like anything else, regrouping needs to follow a CPA approach. Start with concrete objects, moving to pictorial representations next. Adding horizontally should be their last step.
Mistake #3: Lack of Basic Fact Fluency
If you still have students who are counting on their fingers to solve basic math facts within 20, regrouping is going to be a challenge. I get it… it can feel impossible to practice basic math facts daily. Trust me though… it is so worth it to make it a priority!
I fit math fact practice into our math station routine. In my classroom, I have 5 rotations. Stations 1-3, technology, and teacher table. Station 1 is always math fact practice while stations 2 and 3 review previously taught skills. I alternate weeks with addition, subtraction, fact families, adding 3 numbers, etc. As the year goes on, the numbers get harder to keep students learning and growing their math fact knowledge.
Mistake #4: Starting With the Tens Place Rather Than the Ones Place
Starting with the tens place rather than the ones place when solving regrouping math problems is one of the most common problems I see with students, regardless of if they’re adding or subtracting multi-digit numbers.
I think many students get confused because solving addition and subtraction problems is the opposite of the way they’re taught to read. They’re taught to read from left to right, but when solving math problems, it’s the opposite – right to left.
Who made those rules, right?!
We can’t change the rules, so how do we help our students remember the proper direction for solving regrouping math problems?
Teach them to circle or highlight the ones place first. As you teach and model, practice this step repeatedly to show them where they need to start.

In the example of 74 + 49, you can see one of the most common errors students make. In this particular example you cannot tell what order the students solved. It would look the same from left to right or right to left.

Mistake #5: Not Carrying Over the 1 in Regrouping Addition Problems
This regrouping mistake occurs in addition problems, and that is not carrying over the 1 when regrouping is necessary. A student might not carry over the 1 in regrouping addition problems because they don’t fully understand that the value from adding two digits in a column is larger than ten and must be moved to the next place value. This often stems from a lack of conceptual understanding of how place value works and the purpose of regrouping.
When we get to this stage I teach them to ask themselves “if it’s ten or more, carry next door” for every place value they add. If they are able when solving the ones, I have them write the sum next to the problem. They draw arrows to show where each number goes. The ones place of the sum goes in the answer. They draw an arrow to carry the one next door to the tens place. Adding in those simple steps such as drawing arrows can really help.

Mistake #6: Not Adding the Carried Over 1
This next mistake directly follows the previous one. Sometimes, students forget to actually add the carried over 1. When a student forgets to add the 1 they carried over, it’s often because they lose track of the regrouped value during the process. This might happen because they’re focused on completing the next step (adding the tens) and they don’t yet see the carried 1 as part of the total sum.
This can be helped by slowing down, talking through the process while modeling, and circling or highlighting the carried over 1.

Another strategy to try when making this mistake is teaching them how to solve using expanded form. This strategy takes away the aspect of carrying the one over to the tens.
Mistake #7: Carrying Over the Ones Digit Instead of the Tens
When a student carries over the ones digit instead of the tens, it usually indicates a misunderstanding of place value and how regrouping works. For example, in 48 + 17, if they add 8 + 7 to get 15 and write the 5 below while carrying over another 5 instead of the 1, they are treating the digits as separate numbers rather than part of a place value system. This error highlights the need for more practice with breaking numbers apart and understanding that only the tens digit moves to the next column, while the ones digit stays in its place.
Mistake #8: Not Recognizing Regrouping is Necessary
One of the most common regrouping mistakes is when students don’t recognize that regrouping is necessary. This is often because they don’t understand that a single column can’t hold a value greater than nine. For example, in 28 + 26, if they add 8 + 6 and write 14 in the ones place, they may not grasp that 1 belongs in the tens place.

This again points to gaps in their place value knowledge and tells us that they need hands-on activities such as those with base ten blocks to practice regrouping with place value and visualize why regrouping is necessary.
One of my favorite activities to help students see this is my place value work mat, which you can find in my Free Sample Addition with Regrouping lesson plan. Just fill out the form below to have it delivered straight to your inbox.
Mistake #9: Making 10 Instead of Regrouping 10 in Regrouping Subtraction Problems
Another common regrouping mistake is making 10 rather than regrouping 10 in regrouping subtraction problems. Say a child builds out 65 – 17 using base 10 blocks. They need 6 tens blocks and 5 ones blocks. They see that they already have 5 ones, so when they go to regroup, they only count out 5 more ones to make 10 rather than regrouping 10 more.

To fix this, have students line up 10 ones blocks beside 1 tens block before exchanging it. This is a good visual reminder and can also be used for regrouping tens and hundreds.

Mistake #10: Not Being Able to Compare Numbers
Regrouping really is all about those basic foundational skills, and comparing numbers is another key one that students must understand in order to regroup effectively. Many students aren’t able to compare numbers to identify which is larger and which number should be put on top in regrouping subtraction problems.
Again, this goes back to place value. If students don’t understand place value thoroughly, they’ll focus on individual digits rather than the number as a whole. This can mean that they lack the ability to compare numbers.
To help students overcome this, reinforce comparing numbers using place value charts and symbols. Then, move to practicing building the minimum (larger number) with students using base ten blocks or place value discs. Practice subtracting starting with the ones place first. When they do not have enough place value to subtract, they’ll practice regrouping with their manipulatives to get more of the necessary value.
Mistake #11: Regrouping When it Isn’t Necessary
As we start teaching regrouping in our math lessons, students can tend to assume regrouping is needed for every single subtraction problem. Then, they begin to regroup when it isn’t necessary. They might take from the tens when they don’t need to because they don’t fully understand the concept of borrowing and may assume that borrowing is always required when subtracting.
To fix this error, teachers can reinforce the idea of only borrowing when the digit in the ones place is smaller than the number being subtracted.
I like to use this chant with my students to help them remember and assess when regrouping is necessary:
“More on top? No need to stop! Numbers the same? Zero’s the game! More on the floor? Go next door and get 10 more!”
Repeat this chant often and reference it when students are stuck.

Mistake #12: Not Changing the Value of the Number Being Borrowed
When a student doesn’t change the value of the number being borrowed, this tells us that they likely don’t understand how borrowing works in subtraction regrouping problems. They may forget that borrowing decreases the value of the digit in the tens (or hundreds) place by 1, which can lead to incorrect answers.

The best way to help with this is to go back to the basics and reinforce using base ten blocks or place value discs. You could also try subtracting on a number line. This hands-on approach will help students visualize the concept and internalize the process.
Mistake #13: Regrouping Across Zeroes
Regrouping across zeroes is the last regrouping mistake I’ll touch on. When a student struggles with subtracting across zeroes, it’s often because they don’t totally grasp the concept of borrowing across multiple place values.
Look at the example of 307 – 299. Do they understand that 300 is the same as 30 tens? When the zero is throwing them off, teach them to zap the zeroes and make them go away! Take away one ten and give it to the ones place.

How to Teach Regrouping Math Problems and Overcome Mistakes
Whew – that’s a lot of potential regrouping mistakes that you may see as a 2nd grade teacher. I know this can be a bit overwhelming to think of when you’re teaching this challenging skill. However, with these mistakes in mind, you’ll know what to look out for and have tools and strategies in your toolbelt to help your students fix these regrouping mistakes.
Let’s recap a few of the most impactful ways you can help students grasp the concept of regrouping:
- Review! Review! Review! (And then review some more!).
- Use chants.
- Provide visual support.
- Use manipulatives.
- Move from concrete to pictorial to abstract regrouping math problems.
If you’re looking for hands-on, standards-based math lessons to teach regrouping with confidence, check out my Regrouping Bundle (which includes both my addition and subtraction with regrouping math units). You’ll get everything you need to help your students grasp this skill (while getting continuous place value practice) all in one easy-to-use set of lesson plans and activities.
Need more print-and-go options for practicing 2 and 3-digit addition and subtraction with regrouping? Grab these worksheets for easy, no-prep practice. (They’re a teacher favorite!).
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