A good example is Shakespearean english and expressions compared to modern english. It's amazing just how much it's changed. Half the time when someone makes a study of Shakespeare they need reference works in order to understand it properly!
I can recite the prologue to the Canterbury Tales from memory with Middle English pronunciation. It is vaguely understandable but quite obtuse. The language is still changing of course. Much of what you might think of "sloppy" or "poor" pronunciation or "bad English" is in many cases "change in progress".....changes which 100 years from now would not seem in any way unusual. It's always been the case. Lovely, elegent Shakespearian or King James Bible English was thought of as truly dreadful English just 50 years earlier. Many of the quaint endings (i.e. hast, comest, livest, etc.) were still somewhat new in the 1500s and were altering what had earlier been regarded as lovely, elegent English. The future auxiliary will as in He will go home originally meant "will" as in "intend" (that is, He wills to go home). But over the centuries, the meaning subtly shifted and what was a full verb became weaker and weaker until it is just an auxiliary without verbal meaning. That is happening right now with certain other words. In 18th and 19th century English, you would say They have to go home, They want to go home, and so forth. The verbs have and want are real verbs, though slightly weakened, and they introduce infinitive clauses. In formal English, they are still used in this way. But in informal English, in fact, in probably most spoken English, we now say They hafta go home, They wanna go home, etc. What we're now seeing is the emergence of a new auxiliary hafta, which means something close to "must". And wanna means something close to the medieval will (as in, He wills to go home), but it is already becoming an auxiliary -- and if things continue changing in the usual way, in 200 years or so, I wanna go home will mean "I will go home," so it would be totally natural in 2200 AD to say things like "I wanna go home, but only because you're forcing me." After all, saying "I will go home, but only because you're forcing me" would seem totally bizarre to Chaucer's ear.
Leolaia