This chapter presents an in-depth comparative analysis of Thai and English grammar, highlighting the structural, functional, and cultural distinctions between the two languages. The chapter systematically compares core grammatical components—parts of speech, sentence structure, tonal and intonational patterns, idiomatic expressions, and politeness strategies—drawing on both traditional and contemporary linguistic frameworks. Key differences are explored in how each language handles nouns, pronouns, verbs, adjectives, adverbs, prepositions, conjunctions, and interjections, with particular attention to inflectional patterns, word order, and syntactic roles. While Thai emphasizes context, particles, and social hierarchy, English relies more heavily on inflection, rigid syntax, and modal constructions. The chapter also examines how each language constructs questions, negative and compound sentences, and relative clauses, as well as their respective uses of tone and intonation. Idiomatic usage and politeness markers further illustrate the cultural underpinnings of communication. By integrating grammatical description with cross-cultural insights, this chapter offers valuable tools for language learners, translators, and educators navigating the complexities of Thai-English bilingualism.