When is it appropriate to end a sentence in a preposition?

Prepositions end up at the end of sentences that are using phrasal verbs, that is a verb plus a preposition where the preposition isn't starting a prepositional phrase. Phrasal verbs are perfectly okay and are very common in Germanic languages, less common in something like Latin.

English has several registers--levels of formality-- and phrasal verbs are not preferred in formal English.

Above link is dead wayback machine to the rescue until they go out of business.

In my opinion this is one of those stuffy rules touted by grammarians who probably should have better things to do... When you can avoid it, don't end sentences with prepositions, but if rewriting the sentence will make it grammatically tortured, it's best to break the rule for the sake of clarity.

As an example, in casual speech and informal writing it is common to say, "Who are you going with?" but in a speech or formal writing you probably want to use the more grammatically accepted correct form "With whom are you going?"

When is it okay to end a sentence in a preposition?

Any time you please. The "rule" against ending sentences with a preposition is pseudo-Latin piffle with no relevance to modern English in any register.

A bit similar to [Peter's answer], but with some more history:

Supposedly, the saying originated with Winston Churchill, though it certainly could be apocryphal. The original version was

"This is the sort of bloody nonsense up with which I will not put."

... maybe. According to an English professor at Washington State University, there are a lot of variations on that theme out there.

Whatever the details of the story, though, the point is that the preposition rule isn't one that needs to be followed in general use. It doesn't increase clarity or provide any other benefit; it just lets some people provide a different tone to their communication. Or, potentially, feel superior to those who don't obey the rule.

As easily as ending a sentence in a preposition may roll of your tongue in conversation and be comprehended, unfortunately it is not always comprehended as such when read. Often it looks like an unschooled individual is at work here. However, the heart of the matter is that it is a logical distractor, could lead to confusion, and therefore is to be avoided.

You can sometimes cheat by adding "do so" on the end of some cases, I have found. However, it is best to rewrite the material to avoid the logical distractor when necessary.

Now, in an email, depending upon whom you are writing, it is perfectly acceptable in my humble opinion to end in a preposition as long as it is about 90% obvious that the meaning of the sentence will not be lost.

Agree on mocking the pedantry of this rule.

Perhaps the rule had its origin in the idea that every preposition should have an object, and as the object normally follows the preposition in English, if a sentence ends with a preposition that often means that it has no object.

Consider, "Send a letter to." This obvously is an imcomplete thought. Send a letter to whom?

The catch is that sometimes the object does NOT immediately follow the preposition. To take Churchill's famous sentence that others have quoted here, the natural wording of the thought is, "That is a rule I can't put up with." "With" is a preposition and ends the sentence, but it is not the case that it has no object. The object is "rule", which occurred earlier in the sentence. You could, of course, write, "I can't put up with that rule." It doesn't change the strict meaning of the sentence. But it does change the emphasis, from "that rule" to "I", and so is not entirely equivalent.

Attempts to follow this rule lead people to all sorts of awkward constructions. Is it really better to say, "That is the girl with whom I want to go" rather than "That's the girl I want to go with"? How is the first sentence better ... other than that it conforms to an arbitrary rule?

Side note: One violation of this rule that I hear all the time and that really grates on me for some reason is, "Where's it at?" The "at" has no object, because it is completely superfluous. What you want to say is, "Where is it?" I suspect that when people contract "where is" to "where's", the sentence becomes "Where's it?", which sounds too abrupt, so they feel a need to add an extra syllable, and rather than expanding the contraction, they add a pointless unnecessary word.

Yes, it's fine. Even Fowler agrees that the hoary shibboleth forbidding sentences that end in prepositions is hogwash. It came about originally supposedly because such a thing could not happen in Latin, so naturally English must follow suit.

Because all upper-class private schools of the time emphasized, if not required Latin, 'good' grammar was presumed to be grammar that emulated Latin grammar.

But don't take my word for it. Here's what The Oxford Dictionaries has to say:

There’s no necessity to ban prepositions from the end of sentences. Ending a sentence with a preposition is a perfectly natural part of the structure of modern English.

The only time you may wish to avoid ending a sentence with a preposition is when the verb is so far back that its relationship with the preposition becomes unclear.

There is just simply no such rule in English.

Now, in the seventeenth century, Dryden complained about it, because he wanted to write a bit about how he was a much better writer than Shakespeare and Johnson, and the best he could come up with is that Shakespeare and Johnson hadn't followed a rule about preposition position that existed in Latin, but had never existed in English.

On the one hand, that at least is a bit better than the nonsense about split infinitives — where people prohibit something in English not because it's forbidden in Latin but because it's actually impossible in it. On the other hand, it's still pretty stupid.

Ironically, English is overall a much stricter language than Latin about word order, and it's precisely because of this that in Latin a preposition not only can't appear at the end of a sentence, but can not appear after its object (that is indeed the etymology of the word — pre- + position). The idea that this means anything in English only follows if we allow that "means the idea in English anything" is a well-written clause (hey, the preposition's in the right place!).

Following Dryden, some more people followed suit. Now, some would say that it is often more graceful to place the preposition before the object, or that it's a good idea to make sure that the preposition isn't so separated from the words it most closely relates to as to cause confusion. I'll agree with the latter and allow the former as true much of the time, though it can be the graceless option other times.

But some went further and said that there was some magical rule against prepositions.

You might think that today we could easily counter such foolishness by pointing out Shakespeare and the King James Bible use it, and you don't have to believe in biblical infallibility — or indeed believe any of it to any degree — to believe the King James is as infallible as can be when it comes to grammar, with Shakespeare as grammar made flesh. (And Shakespeare clearly isn't infallible in other ways — his geography is hilariously bad and his history allowed clocks to chime in Ancient Rome, but it's bad geography and history, beautifully expressed).

But that was a different era, and such ramblers actually cited Shakespeare as an example of bad English, much as people today may complain that an MTV host saying "yo! We gonna pimp yo' ride sweeeeet!" is a bad example to children learning English today.

Thankfully, Shakespeare won the popularity contest, and we can enjoy his plays in any city in the world, and only rarely are such pontificators of invented grammar rules something we have to put up with.

Okay, sure, it doesn't matter... Unless, of course, you're writing your college thesis and just can't seem to figure out why on earth your lunatic professor only gave your paper a C after all the hard work you put in to it. Hmm... well, maybe it matters just a little, don't you think? I mean, even if you disagree with the convention, try convincing the dean of the university's Linguistics Department how it 'must be correct because so many people use it so often' and see if that gets you anywhere. The same could be said of the word ain't and yet no one insists on its being grammatically correct. Ain't is accepted in conversational English perfectly well but it will be frowned upon when it is put into print. So why don't people just accept the same to be true for prepositional phrase words at the ends of sentences?

See here for the explanation of a prepositional phrase. Particularly, notice the statements "Prepositions are indeclinable words that introduce the object of a prepositional phrase." and "The noun phrase or pronoun that follows the preposition is called the subject of the preposition." these two descriptions make it fairly clear why prepositions don't belong at the end. It's like saying "I'd like to introduce." and then ending the statement there without specifying who you are introducing.

Obviously, if the author is one of literature or prose, or a lyricist, there's good reason why they might want to use improper grammar. Style is often a big part of what draws readers to the content of a piece of work. But that seems to me only to qualify the use of incorrect grammar, not to qualify incorrect grammar as actually being correct.

To be perfectly clear, some words typically used as prepositions are NOT always prepositions! Mistaken identity of the parts of speech is the culprit of every example given in the comments and answers on this page. As the following distinction explains:

2. A word that looks like a preposition but is actually part of a verb is called a particle. Held up is a verb meaning “to rob.” Therefore, up is not a preposition, and bank is not the object of a preposition. Instead, bank is the direct object of the verb held up.
3. Is it a particle or a preposition?  To avoid confusing prepositions with particles, test by moving the word (up) and words following it to the front of the sentence:  Up the bank four armed men held.  If the resulting sentence does not make sense, then the word belongs with the verb and is a particle, not a preposition.

Look, whether or not it should or shouldn't be is irrelevant. The fact of the matter is that the majority of people who are in positions of power in society and industry today, are of the generation of people who were all taught that ending a sentence with a preposition is bad grammar and that generation of people are likely to make a judgement call about people who do so as being, to some degree, uneducated people. So if you really want to be sure to cross your t's and dot i's in any given particular instance, you'd better your chances of making a good impression if you would also avoid the preposition-ending sentences. When the millennials become the majority of people in positions of power in society, then it will become sound advice to encourage neglect of that 'rule'.

The same could be said of the word ain't and yet no one insists on its being grammatically correct. Ain't is accepted in conversational English perfectly well but it will be frowned upon when it is put into print.

This is a non sequitur indeed. The placing of a preposition at the end of a sentence has far more linguistic acceptance (and common usage) than ain't has ever had, or ever will. Does anyone even seriously (non-comically, non-ironically) use ain't anywhere? I'm willing to be so advised.

Rules against sentence-ending prepositions are as forgettable as those against the split infinitive.