Starting With the Basics
How to carry-over on subtraction, or some people refer to it as borrowing, can be confusing to young students. They know how to subtract 15-9 and get the correct answer. But when the problem looks like this 65-29, it becomes more difficult. Many students know you cannot have 5 items and take 9 away without getting a negative number. However, when they are working an entire page of subtraction problems, they may decide to do 9 - 5 instead of using the carry-over or borrow strategy. So, what do you do?
First, start with the basics of how to carry-over on subtraction by teaching students that you always subtract top to bottom. Because we teach them that with addition, it doesn't matter which way you add the numbers (5+6 or 6+5), they may believe that same rule is true with subtraction. Do several easy problems with students--maybe have students work problems on the chalkboard--to show that with subtraction, you start with the top number and subtract the bottom number.
You can get out manipulatives for each student and ask them to put 5 blocks on their desk. Now, ask them to take 9 away. They will easily see that this is impossible.
Before students learn how to carry-over on subtraction, they need to understand these basic subtraction rules--it doesn't hurt for any student having difficulty to review these during one class period.