1. In job interview, during the formal session, use "yes", during the informal session, e.g., interview lunch, use "yeah". 2. On the job, in a meeting when your boss is discussing issues, use "yes" to show you are in problem solving status; working with your peers, use "yeah" so to keep them working with you. 3. Talking to your girlfriend, ... "yes"? "yeah"? ..., hum, vital to the relationship ...