What is Geoffrey Chaucer's influence on English and English literature?

This is a huge question - and people have written whole books in order to answer it. I can provide you with the main points though:

• Chaucer considerably expanded the word-stock of English, being one of the first poets in the language to utilise its tremendous variety, bringing in words from a variety of languages which were converging with English during the Middle Ages. Chaucer's "first instances" of words include words from Greek, Latin, Arabic, German and French - and the following regularly-used words: acceptable, altercation, annoyance, arbitration, army, arrogant, arsenic, arc, and aspect.
  
  
• Chaucer made several metrical innovations to the way poetry and verse were written in English.
  • He was one of the first poets to consistently break out of the medieval alliterative tradition and write in accentual-syllabic metre (lines constructed around both the number of syllables and where the accents on those lines fall).
    
  • Chaucer was one of the first to use the five-stress line, which led the way to the iambic pentameter of Shakespeare adn Marlowe. "The Legend of Good Women" is one of the first times five-stress lines appear in rhyming couplets - a form which then became a norm in English poetry.
    
• You can also see the influence on Chaucer on a whole load of writers. To give one example, Shakespeare's "Troilus and Cressida" owes much to Chaucer's "Troilus and Criseyde".