While you type you may make mistakes and hit the wrong key. In that case the student's line will change its color (per default to red) and you have to hit Backspace to erase the wrong character (or characters) again.
Mentres escribes pódeste trabucar e premer a tecla equivocada. Neste caso, a liña do estudante muda de cor (por omisión para vermello) e tes que premer a tecla de borrado para tras para borrar o carácter ou caracteres nos que te trabucache.