Old English (or Anglo-Saxon) was, compared to Modern English, a heavily inflected language. That is, the function of a word in a sentence was indicated by the endings: Se hund biteÞ Þone ealdan mann (the dog bites the old man) means exactly the same as Þone ealdan mann biteÞ se hunda (the "Þ" stands for "th") or (the more common syntax) Þone ealdan mann se hunda biteÞ. In Modern English the position of the words determines their meaning; "The old man bites the dog" differs considerably from "The dog bites the old man." In Old English the -ne on Þone and -an on ealdan indicate that "mann" is the object of the action, no matter in what order they appear, and the forms of Se and hund indicate that the "hund" does the biting.
However, even in late Old English times word order was becoming dominant and in the following years the grammatical endings became less important. For whatever reason a regular change took place: Final unstressed vowels moved first to schwah (the sound in the middle of "telephone") and then to zero, when they became silent. The ending of the word "tale," for example moves through these stages, from the sound "oo" to schwah to silence:
Old English: talu > Middle English tale > Modern English "tale" (Click for sound )
Vowels within inflected endings (such as -ode and -as) moved the same way:
Old Eng.: lufode > Mid. Eng. lovede, loved > Mod. Eng. "loved" (Click for sound
Old Eng.: stanas > Mid. Eng. stones > Mod. Eng. "stones" (Click for sound )
A final -n slowed the process somewhat, and so -an survives from Old English in Middle English as both -en and -e:
Old Eng. bringan > Mid. Eng. bringen, bringe > Mod. Engl. bring (Click for sound
The relative tenacity of the final -n (which survives in Mod. Eng. past participles ("broken promises") accounts for Chaucer's final -e in weak (or definite) adjectives; the -e was lost on strong (or indefinite) adjectives, but retained on the weak (which are used after an article, possessive, and such):
Old Eng. Strong: geonge cniht > Mid. Eng.: yong knight > (Click for sound )
Old Eng. Weak: Þone geongan cniht > Mid. Eng. The yonge knight (Click for sound )
There are many more complications in the history of the loss of final -e. However, this may be sufficient to show that there is an orderly process in the evolution of the forms Chaucer used.
It is worth noting that this aspect of Chaucer's verse was unknown for centuries. By Shakespeare's time the final -e had been lost. That is why, though Shakespeare's pronunciation differed from our own, it is possible to read his works in a modern pronunciation: the rhythm of his lines remains the same, no matter how the vowels are pronounced, because except for a few exceptions ("Out damnéd spot!"), Shakespeare treated what had become in his time the "silent e" in the same way we do. Consequently, when Shakespeare read Chaucer he omitted the final -e, treating it as silent. The meter was ruined; though Shakespeare greatly admired Chaucer, he and his contemporaries thought that Chaucer was an archaic poet who could not write a smooth and pleasing meter in those distant early times. So too did John Dryden, who idolized Chaucer but thought he wrote in "the infancy of our Poetry". Not until the the late eighteenth century did scholars discover and demonstrate the importance of the final -e for Chaucer's versification.