Grammatically speaking, ending a sentence with a preposition (on, in, for, of, as etc.) is against the rules and the bane of English language pedants’ existence (along with stray apostrophes and young people in general). But why?
When encountering an unlabeled envelope, do they ask, “For whom is this letter?” or do they ask, “Who is this letter for?”
I truly hope it is the latter, for ‘tis the only way they will get a response that doesn’t comprise an undertone of mockery.
There is also no particular reason for this rule. Most rules in grammar are in place to ensure clarity of meaning, yet this rule adds no clarity. So, why does it exist?
The most likely answer is that in the seventeenth century, English scholars who were involved in the development of the first books on English usage were also Latin lovers (the language that is, not our Latin American friends). Within the confines of Latin, there is no way to end a sentence with a preposition. As many of the rules for English usage come from Latin, it is most likely that the seventeenth century ancestors of today’s pedants decided that if you can’t do it in Latin, then you shouldn’t do it in English.
Having said all that, there are still times when you probably should follow the rule. Very formal writing, and writing for universities and scholars should probably follow the rule book. But, if you’re ever criticised for ending a sentence with a preposition, feel free to tell the criticiser that Shakespeare, Jane Austen, James Joyce and many more classic literary authors have all broken the rule without damaging their reputations.