The goal of this work is to make human-machine interaction in natural language smooth and efficient. To realize this, the machine has to generate natural language sentences which are understood by users most easily. And it is important for this understandability that each sentence is appropriate to the conversational situation at the time of its utterance. In this paper, we will shed light on this appropriateness and consider how we can formalize the method to generate such appropriate sentences. First, we will define such appropriateness; second we will discuss some surface features of Japanese sentences and consider how people decide to add such features to their sentences depending on situational factors. Then we will show brief methods of deciding how to add suitable surface features to sentences in suitable situations. Finally, we will show an implementation of a conversational system which adopts these methods.