Good teaching notices the moment a student starts pretending to understand.

There is a moment when a student stops following and starts performing. The nod comes a beat early, the copying continues neatly, the questions dry up, and from the front of the room it can look like things are going well. Everything after that is wasted, layered on ground that gave way earlier. Catching the switch matters more than any explanation that follows it.

Watch for the nod that arrives too fast and the writing that keeps going with no thinking in it. Stop and ask them to explain the last step back to you. Make being lost easy to admit by saying which part usually confuses people.