Based on formal and functional understanding of linguistics, this study illustrates the necessity of acquiring the functional aspects in language use into English translations. First, we talked about the limitations of formal approach to English passive voice for English translation, pointing out that this approach mainly focuses on the analysis of form-oriented sentences. For a better understanding of the passives, the concepts of functional analysis beyond the level of sentence is required in grammar translations from English to Korean. To examine the applicability of functional grammar of English translations to Korean, we explored the usage of passive voice sentences in The Great Gatsby by F. Scott Fitzgerald and its Korean translations. For this, a selection of English passive voice structures from The Great Gatsby and its Korean adaptations in parallel were discussed, to analyze the use of active voice, passive voice, and thematic-coherence appropriateness in usage and for translations. The findings of the functional analysis shows that functional approach of translations of English could improve the understanding of the passive voices in translations because it incorporates not only the form and meaning aspect of the language, but also the usage. Finally, we make some suggestions for the improving translations by incorporating functional grammar into the process.