1. Used to introduce a greater or more extreme case following a lesser one; expressing that if something is true for a smaller case, it would certainly be true for a larger one
He can't even speak English, let alone French.
Ele nem consegue falar inglês, que dirá francês.
2. Used rhetorically to emphasize an unlikely or impossible scenario
A poor student passing the exam? What about becoming a doctor?
Um aluno pobre passando na prova? Que dirá se tornar médico?