When Chaucer grew up, the educated class was bilingual with the literature written in French since the wealthy and educated class were mostly decendants from the Norman lords who, along with William the Conqueror, disposed Harold of Hastings, slaying his nobles along with their leader.
So, from 1066 until Chaucer's time French was the dominant language for literature. But, when Chaucer wrote, he gave preference to English. Of course, his "Canterbury Tales" is exclusively in English, and since it became so popular, Chaucer's use of this language fostered its popularity. Thus, Chaucer has been credited with the title "Father of English Poetry."
In a way, Chaucer's writings have a lot in common with modern tabloids. Just like with The Enquirer and other gossip magazines, many more people in Chaucer's period read his work than were willing to admit to it. At the time, the two primary languages were that of the aristocracy and the church--French and Latin, respectively. No one who was anyone would be caught dead reading the language of the poor and conquered Anglos--English. As a favorite of the king--as can be seen in the many royal appointments he was given and the ransom paid for his release when held prisoner by an enemy power--Chaucer made a conscious decision to promote the use and acceptance of Middle English by writing his works in English. That decision, coupled with his brilliant style and gift for irony, is why he is known today as "The Father of English."
Geoffrey Chaucer, often called the father of English poetry, made the English language respectable. Before Chaucer, it was not fashionable for serious poets to write in English.
Chaucer established rhyming couplets as the main narrative system in English. During Chaucer's lifetime there was a serious attempt to re-establish the old alliterative system as the main narrative method in English poetry. It was mainly thanks to Chaucer that English adopted the rhyming system. Chaucer was a major force in establishing the dominant meter for English poetry, and in introducing rhyme as the main way of writing a poem. Chaucer more than any other writer is the reason English literature turned out the way it did.
Also by composing in the vernacular – the everyday language spoken in London and the East Midlands – Chaucer lent respectability to a language that would develop into the medium for one of the world’s greatest bodies of the literature. In this sense he is indeed the father of the English poetry.
Read the following passage by John V. Fleming
By any sensible assessment, Chaucer is among the greatest of world poets -- by turns funny, solemn, deeply religious, and ribald. He is among the most technically varied and accomplished of English poets -- meaning that it is possible to learn a great deal about poetry from relatively short passages of his text. He is preeminently the master of the greatest neglected genre in our literature -- narrative verse, poems that tell stories.
Although the question of language is not the only or even principal question that needs to be addressed, it is obviously an intimidating one for students and teachers alike. Chaucer's English is very different from ours, but any intelligent student will, with a few moments' patience, recognize it as English. The same cannot be said of the language of Beowulf. The Course Description describing the AP English Language and Literature course does suggest that some knowledge of the historical development of our language is a necessary part of literary study, and while we cannot expect high school students to become Germanic philologists, we can expect them to comprehend that "April" might once have been spelled "Aprille." It is possible, of course, to read Chaucer in a modernized text, just as it is possible to take a bath with your socks on.