Why was Geoffrey Chaucer's English considered relatively modern for his time?

Geoffrey Chaucer is often described as the first finder of the English language because his work was remarkably modern compared to that of his contemporaries.1

His status as a pioneer is largely attributed to his deliberate choice to write his literary masterpieces in English rather than in Latin or French, which were the dominant languages of learning and the aristocracy at the time. By utilizing the East Midland dialect—the speech of London—Chaucer helped elevate a vernacular tongue into a sophisticated medium for literature.12

Furthermore, Chaucer introduced iambic pentameter to English verse. This metrical structure, consisting of 10 syllables arranged in five metrical feet, became the predominant meter in English poetry by the 16th century, demonstrating the lasting impact of his stylistic innovations. His ability to blend complex philosophical inquiry with humor and a keen observation of human nature allowed his writing to transcend the linguistic limitations of the 14th century, setting a foundation for the development of modern English literature.31