English has its roots in the Germanic languages, yet today it is not even a shadow of its former self. Over the course of history, it has been transformed dramatically through invasion, colonization and social and cultural change. Today, English continues to be in a constant state of flux, as are all languages. The only languages not in perpetual flux are dead languages. It's simply the nature of language. Language changes as our society and culture changes. There are many reasons for this, but I'm by no means an expert on the subject, and I won't try to be. There are plenty of great articles on the topic already.
I am often struck by just how much our language has changed when reading the classics. Shakespeare can be very challenging to read, sometimes even seemingly foreign, yet that was the way people spoke at the time; well the language any way, not the poetic verse! Jane Austen, Charles Dickens, Edgar Allen Poe, and Mark Twain are undeniably some of the greatest writers of the 19th century. Their writing is easier to read, yet in the century plus that has passed since their pens flowed, it still takes a bit more effort to fully comprehend them today than it would have during their era. Contemporary writers such as Stephen King, J.K.Rowling and John Grisham are understood in an instant because they write in the same language we speak.
I'm often fascinated by and drawn into the debate as to whether the English language is changing for the worse. I would have to say no, I don't think so. It's not the language that is at issue, but rather it's usage; grammatical abuse in particular. Funny thing is, Shakespeare, Poe, Austen and the myriad of great writers over the centuries have had their critics too. Geoffrey Nunberg put it best when he said in 1983,
"Our picture of the English of previous centuries, after all, has been formed on the basis of a careful seletion of the best that was said and thought back then; their hacks and bureaucrats are mercifully silent now."
Yes, we abuse grammar, but every generation has done the same. Hopefully the 'careful selection' of English we leave behind for future generations will make us look good too. Only the cream of the crop!