A good martial arts instructor informs students of how they’re doing in a constructive manner. Don’t single out a student who isn’t doing something correctly.
Instead, address the whole class with pointers. Chances are, the student who needs the instruction will catch on. Avoid negative words like “wrong” and instead focus on how the students can improve. Everyone responds better to positive reinforcement.
It always helps to demonstrate the moves again, too, so students can get a refresher on how the correct form looks.