What was Chaucer's contribution to English literature?

In the preface to a publication of The Canterbury Tales, editor D. Laing Purves writes,

Perhaps in the entire range of ancient and modern literature there is no work that so clearly and freshly paints for future times the picture of the past; certainly no Englishman has ever approached Chaucer in the power of fixing for ever the fleeting traits of his own time.

In or around 1378, Chaucer began to develop his conception of...

In the preface to a publication of The Canterbury Tales, editor D. Laing Purves writes,

Perhaps in the entire range of ancient and modern literature there is no work that so clearly and freshly paints for future times the picture of the past; certainly no Englishman has ever approached Chaucer in the power of fixing for ever the fleeting traits of his own time.

In or around 1378, Chaucer began to develop his conception of writing poetry in English, a language that would be linguistically accessible to all. This use of English would be obedient neither to the court, whose official language was French (Chaucer had written poetry himself in French) nor to the Church, whose official language was Latin.

Arguably, then, one of Geoffrey Chaucer's most significant contributions to English literature is his having written his magnificent work, The Canterbury Tales, in the language of the English people. That the two powerful forces of the court and the Church were bypassed for the sake of writing in English, making his work more accessible to the people, afforded Chaucer's work more widespread appeal, and it created a remarkable portrait of medieval England. In what is considered by many to be his magnum opus, Chaucer revealed the changes taking place in the English language and in society as a whole. His work also stands as a significant achievement in Middle English verse.