Considered the preeminent English poet of the Middle Ages, Geoffrey Chaucer was well versed in other languages; in fact, he translated two tremendously influential works from Latin and Old French into Middle English. And, while he was not veritably the only one who wrote poetry in English, he was extremely influential in promoting English from something other than the "vulgar" language that it was considered to be. His poetry that reflects Latin, Italian, and French sources--he wrote much French poetry--evolved into the verses that he wrote for The Canterbury Tales. Clearly, his remaking of French, Latin, and Italian sources and treatment of secular and religious allegory into his delightful tales brings the vernacular of English in The Canterbury Tales to a new respectability. Perhaps, then, for this reason, that he brought legitimacy to the vernacular English when the dominant literary languages were Latin and French, Chaucer is somewhat mistakenly credited with being "The Father of English Poetry."
With the publication of The Canterbury Tales in English, then, Chaucer made a strong contribution to shaping English literature. With English, Chaucer realistically shaped the speech of the pilgrims while also realistically satirizing their manners, thus creating what was to become a popular literature among people of the same social types. In short, Chaucer brought literature to all levels of society, not just the nobilty and elite.
In addition, Chaucer experimented with different forms of verse,
...establishing a decasyllabic line that, to become the iambic pentameter of the sonnet, blank verse, and heroic couplet, is English poetry’s most enduring line.
This experimentation which left a lasting mark upon English poetry is a contribution of Chaucer's that is also cause for considering him the father of English poetry.