If somebody wants to know whether we understand her or no
she would ask:
¿Me entiendes? – Do you understand me?
¿Me oyes? – Can you hear me?
¿Entiendes español? – Do you understand Spanish?
In order to express your misunderstanding you can use the following expressions:
No entiendo – I don´t understand
Te oigo muy mal – I can´t hear you well
Más despacio, por favor – More slowly, please
¿Puedes repetirlo, por favor? – Can you repeat it, please?
No hablo español – I don´t speak Spanish