Answer:
Old English (up to about 1200) has an exclusively Germanic vocabulary, complex declension systems, a completely separate set of pronouns for two people as opposed to one or three, and odd syntax. Readers of Modern English cannot read it without training and a good glossary. We would have a hard time understanding it if it were spoken to us, too. Middle English (1200-1400) is easier to read--most Modern English speakers could make out what Chaucer wrote without too much trouble. However we might have a very difficult time understanding it if it were spoken to us. The change to modern English involved a shift in vowel sounds (unimaginatively called the Great Vowel Shift) which made English sound like it does today, although it is still spelled like Middle English. Of course there are and always have been accents and regional dialects but the core of the language is the same as it was when Henry VIII spoke it.