This study is a detailed investigation of a number of issues in colloquial Sinhala morphosyntax. These issues primarily concern grammatical relations, argument structure, phrase structure and focus constructions. The theoretical framework of this study is Lexical Functional Grammar.¶Chapter 1 introduces the issues to be discussed, followed by a brief introduction of some essential aspects of colloquial Sinhala as background for the discussion in the following chapters. In Chapter 2 we present basic concepts of the theoretical framework of Lexical Functional Grammar.¶ The next three chapters mainly concern grammatical relations, argument structure and clause structure in colloquial Sinhala. Chapter 3 examines grammatical relations. The main focus lies in establishing the subject grammatical relation in terms of various subjecthood diagnostics. We show that only a very small number of diagnostics are reliable, and that the evidence for subject is weaker than assumed previously. All the subjecthood diagnostics that were examined select the most prominent argument in the argument structure as the subject, i.e. 'logical subject'. However, there appear to be no processes in the language that are sensitive to the subject in the grammatical relations structure, i.e. 'gr-subject'. Further, there is no evidence for other grammatical relations like objects. In Chapter 4 we discuss the agentless construction and related valency alternation phenomena. It was previously assumed that the agentless construction, valency alternation phenomena and the involitive construction are all related. We argue that the agentless construction should be treated as a different construction from the involitive construction. We also show that the agentless construction and the involitive construction have contrasting characteristics, and that treatment of them as separate constructions can account for some phenomena which did not receive an explanation previously. The valency alternation phenomena are related to the agentless construction, therefore there is no valency alternation in involitive constructions. It will be shown that verbs undergoing the valency alternation can be distinguished from the other verbs in terms of the lexical semantic properties of individual verbs. Chapter 5 examines the structure of non-verbal sentences in terms of a number of morphosyntactic phenomena. It was previously argued that verbal sentences and non-verbal sentences in colloquial Sinhala differ in terms of clause structure. However, the present study shows evidence to the contrary.¶ The next two chapters deal with modelling contrastive focus and the phrase structure of the language. Chapter 6 is a detailed analysis of the contrastive focus (cleft) construction in various clause types in the language, and proposes a unified syntactic treatment of contrastive focus. Contrastive focus is in some constructions morphologically encoded, while in others it involves both morphological and configurational assignment of focus. The complex interaction between focus markers and verb morphology in various focus constructions is accounted for by general well-formedness conditions applying to the f-structure, and the principles of Functional Uncertainty and Morphological Blocking. In Chapter 7, we discuss the phrase structure of the language, in particular such issues as its non-configurational nature and the lack of evidence for VP. We propose non-configurational S and some functional projections to account for word order freedom under S and to explain certain morphosyntactic phenomena, such as configurational focus assignment. Finally, Chapter 8 summarises the conclusions made in previous chapters.