Teaching Borrowing (Subtraction Regrouping)
This is a tough concept to learn for students with directional, spacial, or place value weakness.
It took 25 years of teaching regrouping to students with all kinds of manipulatives and visuals and strategies before a student shared a method she learned in her class. Once the concept has been taught and retaught, it can then be practiced with an easy storyline that takes all of the confusion and reversals out of the equation. "I have 4. You want 6. Do I have enought? No. Go next door and borrow. Scratch it out...etc...etc"
After a few days with the storyline it is memorized and the guesswork is gone!