This paper addresses a scenario where a smart environment drives a proactive robot to motivate the elderly in maintaining a healthy lifestyle. The robot is in charge to trigger a vocal interaction with the person by proposing a suggestion that is polite and persuasive. We focus on the representation of sentences based on some attributes and, through spatial and temporal reasoning, we classify them over time. In an ontology, attributes are used to classify sentences and decide if the robot should start an interaction, and with which sentence. We present a preliminary implementation of the system, and we show that it is suitable for undertaking an iterative development process guided by domain experts toward a polite and persuasive virtual coaching.