Geoffrey Chaucer became known as the "Father of English Poetry" (or literature), primarily due to his groundbreaking work "The Canterbury Tales," which included a number of forms that went on to shape the English language for years after it was initially written. Chaucer never completed "The Canterbury Tales," and continued to work on it until his death in 1400.
One of the most innovative aspects of "The Canterbury Tales" is that it was written entirely in English vernacular. At the time of writing, almost all literature was written in Latin or Anglo-Norman. Using the language of the every day immediately brought Chaucer's work to the masses.
Chaucer was also the first to introduce the stylistic form of the couplet. This later evolved into the heroic couplet, a form often used later in epic English poetry. Additionally, Chaucer was the first to incorporate the regular use of iambic pentameter in his prose. This laid the foundations for almost all subsequent prose writing, as well as many poetic forms, which often include iambic pentameter as their primary rhythm.
Chaucer was heavily influenced by early French romantic poetry, some elements of which he went on to incorporate in "The Canterbury Tales," which can be seen in "The Second Nun's Tale."