The temporal interpretation of events is crucial to many applications of natural language understanding. Interpreting events as described in individual utterances and discourse requires both semantic and pragmatic processing. We develop representations for events that reflect the internal structure of different situation types. We propose semantic constraints through which the representations of different events can be related based on their tense, aspect, and situation type. Our approach uses semantic information to a greater extent than previous approaches, and thereby generates constraints that restrict the ultimate pragmatic interpretations. We develop our approach on the notorious when.clauses, and show how it may be used for understanding events in discourse.