To say 'I don't understand' in Czech, you can say 'Nerozumím' or 'Nevím' (I don't know). These phrases can be used when you are having difficulty comprehending something or need further clarification. It is common to encounter new words or expressions while learning a language, so don't hesitate to ask for explanations or repetitions when needed. Native speakers will appreciate your effort to communicate and will often be willing to assist you in understanding.