The standard non-scholarly answer is that people were lazy and careless with their use of language. Certainly some people have been (and still are) guilty of lazy and careless usage. But are we then to conclude that we have inherited 1,500 years of laziness and carelessness, and that, if we were only more conscientious, we could all speak beautiful Old English? The fact is that even competent use of the language has changed over time. The more discerning of you might come up with a different reason for the changes English has undergone: to simplify its many complexities. Well, if that’s what you thought of, you’re on to something. But you shouldn’t think that we now speak a simpler language than the Anglo-Saxons did. Tweaking one aspect of the language to make it simpler often creates new complexities. This is what drives language to change constantly.
Posted in: FAQ for English 400: History of the English Language