This chapter explores the grammatical functions and communicative significance of adverbs and prepositions in the Thai language. Adverbs, categorized into four main types—ordinary, specific, interrogative, and temporal—are examined in terms of their roles in modifying verbs, other adverbs, or entire clauses. Each type is supported with authentic examples illustrating nuances in manner, frequency, degree, and time. The chapter also highlights idiomatic expressions and figurative adverbs, demonstrating their cultural resonance and rhetorical impact in Thai discourse. Prepositions, meanwhile, are analyzed for their role in establishing syntactic and semantic relationships among sentence elements. This includes spatial, temporal, causal, purposeful, possessive, comparative, and instrumental functions. Special attention is given to prepositional phrases, polysemy, and syntactic positioning, as well as to words related to determination and numbers, which further shape noun phrases. Drawing from both traditional grammar and modern linguistic theory, the chapter offers a comprehensive framework for understanding how adverbs and prepositions enhance clarity, specificity, and expressiveness in Thai communication.