This is a great essay topic. Let's brainstorm some ideas and examples that will contribute to an understanding of the changes in English over the centuries.
The first thing to remember is that English developed in stages. The language that would become Old English was brought to England by tribes like the Angles, Saxons, and Jutes that migrated to the island from Europe in the 400s or so. Old English (with its several dialects; it was not spoken or spelled uniformly at all) was spoken until the later 1000s and early 1100s, when it gradually shifted into Middle English. Modern English developed after the time of Chaucer and the Great Vowel Shift that took place between 1400 and 1700. None of these transitions were abrupt; they happened gradually over time as speakers naturally shifted their usage of the language.
Let's look at some examples of spelling changes over the years. In Old English, “book” is spelled “boc,” and “books” appear as “bec.” The vowel in the latter represents a sound shift in some Old English nouns. If we look at the Old English sentence “Sē cyning giefþ þǣre cwēne bēah,” (“The king gives the queen a ring), we especially notice the shifts in spelling that happened over the centuries as the language developed.
We can see the differences in spelling between Middle English and Modern English by examining the first line of Chaucer's Canterbury Tales prologue: “Whan that Aprill with his shoures soote,” “When April with his showers sweet.”
Vocabulary also both expanded and shifted in meaning as English developed. The Old English word for “dog,” for instance, is “hund.” The Modern English version is “hound,” and it refers to only a particular type of dog rather than all dogs. The Old English verb “magan” developed into the Modern English “may,” but it has a different meaning. “Magan” means to be able to do something and also to be strong or sufficient or to prevail. Modern English “may” refers to permission to do something; it has a much narrower range of meaning.
In terms of grammar, Old English is a much more inflected language than Modern English. This means that its words have many more forms that express their role in a sentence. Modern English, for instance, has the definite article “the,” just one word. But Old English had a whole spectrum of words that inflected to match the case, number, and gender of the noun they accompanied. We can see that from the sentence above in which both “sē” and “þǣre” mean “the.” Nouns, adjectives, pronouns, and verbs also have many more forms in Old English than they do in Modern English or even Middle English. Middle English still has more than Modern English but significantly fewer than Old English. The language's grammar simplified over time.