must not vs don't have to

Question

Hola. How do express the difference between "The students must not use a calculator" (in the examples), and "The students DON"T HAVE TO use a calculator." which has a very different meaning. Thank you!

Inma Sánchez · Accepted Answer

Hola Ann
- When you mean: they don't have to as in it is their option to use it or not but they don't have to, you'd say:
Los estudiantes no tienen que usar una calculadora. 
or
 
Los estudiantes no tienen por qué usar una calculadora (si no quieren).
 
- For the obligation to not use a calculator as in it is not permitted, you'd say:
Los estudiantes no pueden / no deben usar una calculadora. 
I hope this clarifies it.
Saludos y felices fiestas
Inma