The thesis is an attempt to give a precise formal semantics for various time-referential linguistic categories of English such as tense, perfect, progressive and Aktionsart or "action type", with the ultimate goal of explaining why with a verb phrase such as walk the inference from, for instance, John is walking to John has walked is intuitively valid, while with a verb phrase such as build a house the even weaker inference from John is building a house to John will have (...) built a house is intuitively invalid. The difference is seen as one of the Aktionsart of the sentences involved. ;Accordingly, a theory of Aktionsarten is developed in chapter 2. Four different Aktionsarten are formally distinguished. In chapter 3 a theory of the tenses and their interaction with temporal adverbials is worked out. Chapter 4 is about the perfect. The so-called "extended now" theory of the perfect is given a precise treatment. Finally, chapter 5 deals with some problems of the English progressive. Especially the so-called "imperfective paradox" is given a solution. ;The method I use is that of model-theoretic semantics applied to natural languages, as it was first developed by the logician, Richard Montague. (shrink)