The transition from Old English to Modern English was a process, not an event
Changes in language don’t occur overnight, though slang terms come in and out of use relatively quickly and new words are invented while others fall into disuse. The rules of grammar you learned in school are the same ones your parents were taught and what your own kids will (or do) use. A few new words are tossed in the mix every few years to keep things interesting (remember the uproar when “ain’t” was added to the dictionary?).
The transition from Old English to Middle English to Modern English was a process rather than an event — the rules didn’t all suddenly change on May 24, 1503. Before the Normans invaded England in 1066, the people living in Britain spoke Old English or Anglo-Saxon. Some of the words from that time are still with us — the ones of the vulgar four-letter variety. Old English was so unlike Modern English it’s fair to view it as a foreign language. For example, here are the opening lines of the poem Beowulf:
Hwæt! Wé Gárdena in géardagum
þéodcyninga þrym gefrúnon
hú ðá æþelingas ellen fremedon.
I’m completely lost. Something about a garden, maybe?
Modern English translation as follows:
Listen! We — of the Spear-Danes in the days of yore,
of those clan-kings — heard of their glory,
how those nobles performed courageous deeds.
Yeah, not even close.