Vocabulary: Many words that were commonly used 600 years ago are no longer used today, and many new words have been added to the language. For example, the word "computer" did not exist 600 years ago, but it is now one of the most commonly used words in the English language.
Grammar: The grammar of the English language has also changed over time. For example, the use of the verb "to be" as an auxiliary verb (as in "I am going") is a relatively recent development.
Pronunciation: The pronunciation of many English words has changed over time. For example, the word "knight" was pronounced "knikht" 600 years ago, but it is now pronounced "night".
Spelling: The spelling of many English words has also changed over time. For example, the word "though" was spelled "thoughe" 600 years ago.