This dissertation presents a study of three linguistic areas--morphology, syntax and semantics--of what have traditionally been called auxiliaries or auxiliary verbs in Thai, but what I call temporal verbs. My morphological analysis offers answers to long-term questions: What is the grammatical category of temporal verbs? What is the structure of sequences of these elements? And how are their syntactic discontinuities to be handled? My syntactic analysis investigates all possible positions of temporal verbs in both Subject-Verb-(Complement) and Verb-Subject-(Complement) sentences (Sookgasem 1989). Using Head-Driven Phrase Structure Grammar (Pollard & Sag 1987) for my analysis, I focus on three interesting points: First, a problem with the Head Feature Principle when describing a temporal verb in a sentence. Second, a problem with the linear order of the VP constituent in the Verb-Subject-(Complement) constructions of temporal verbs. And third, a position of some temporal verbs in relation to part of its VP complement. For the semantic analysis of temporal verbs, I focus on the temporal interpretation of the Future and Aspect verbs. I argue that Thai is a tense language. To support this, I analyse Aspect in Thai and examine tense interpretation in simple sentences and all types of complex sentences. Based on the evidence, I propose a tense system in Thai. I provide definitions of Reference Time and Tenseness. I propose a Tense Assigner Hierarchy, a relation named Overlap, a semantic model for tense interpretation, and the truth conditions for tensed sentences and clauses. To provide an accurate account of tense interpretation in Thai, I analyse eventualities which include Activities, Accomplishments, Achievements and States.