One Grammar Rule You Need to Know to Break

This is the sort of English up with which I will not put.
--Attributed to Winston Churchill, in a note to an editor who clumsily rearranged one of Churchill’s sentences to avoid ending it in a preposition.

The rule that forbids ending sentences with prepositions is showing its age in modern speech and writing. You're going to have to know when to use--and when to abuse--this increasingly archaic rule.

If I had to give it a firm date, I'd say this rule died in 1996 with the release of one of history's most puerile movies:

From the screenplay of Beavis and Butthead Do America:
ATF Agent Bork: Chief, you know that guy whose camper they were whacking off in?
ATF Agent Flemming (appalled): Bork! You are a federal agent! You represent the United States Government! Never end a sentence with a preposition! Try again.
Bork: Oh, uh... You know that guy in whose camper they... I mean that guy off in whose camper they were whacking?
Flemming: That's better. Yes?

Sure, if you're speaking or writing to an audience of English literature professors, you should place a premium on observing this rule, even if it means turning your sentences into syntactic pretzels. But if your audience consists of normal people who just want to understand you, don't be such a stickler for grammar.

Otherwise you'll just sound, well, like a government agent.