What is Chaucer's contribution as a poet and as author of The Canterbury Tales?
Chaucer's contributions to poetry are evident in the title that posterity has bestowed upon him: Chaucer is the “father of English poetry.”
Before Chaucer, many of England's literary works were written in other languages, Latin or French or earlier forms of English. What Chaucer did was take the developing language of Middle English and create poetry, most famously in The Canterbury Tales, that described the varying elements of English life—knights, doctors, priests, farmers, merchants, etc., and he did it so well that others in Europe took notice, thereby popularizing the English language for the first time.
English had been evolving for centuries, incorporating Anglo-Saxon and French influences. In Chaucer's time, it began to stabilize and develop on its own, finally free of significant outside influences. Chaucer didn't invent the English that he used, but he did take it to a new plane. After Chaucer, English was no longer considered a substandard language, subservient to other languages. Now it stood on its on, continued to develop, and soon a new guy showed up and started writing, in early Modern English, what many consider to be the best literature ever written—William Shakespeare.
Chaucer's indebtness to French poetry cannot be mitigated. Expressing no interest in older English forms, the Roman de la Rose was the primary influence in Chaucer's poetry as he translated this and he himself composed French poetry. This influence of French poetry is evidenced in The Canterbury Tales with certain niceties of rhyme, and his not rhyming words ending in -y with others ending in -ye. Also evident is the influence of older stress lines and French syllable counting.
Clearly, Chaucer's poetic virtuosity and use of the French verse romance and rhyme royal in which he emplyes the style of French poetry and does not begin with a straightforward narrative had a great effect.