If there's something you don't understand, it's always good to ask. The professor answers the questions about past episodes that Radio D listeners have sent in. The listeners ask and the professor answers, getting to the root of every question. It's a good opportunity for the listeners to review information, expand their knowledge, or just ask something they always wanted to know. Listen for these question from the listeners -- and the professor's answers to each of them: Which address is suitable for which situation? When can I use "du" or "Sie"? How do I introduce myself? When do I know when to use first or last names? What do modal particles like "denn," "doch," and "eigentlich" mean? What is the difference between "nicht" and "nichts"?