Mientras versus cuando
When do you use which?
No. Sometimes they are even more or less interchangeable: "Escucho música cuando hago deporte" and "escucho música mientras hago deporte" means the same thing.
Let's try another explanation. Try to think of "cuando" as "in the moment(s) that" and "mientras" as "while in the progress of". It sounds rather artificial using it in English, but maybe it will help you understand the concept. "Mientras" implies that somethings is in progress of happening.