You must never end a sentence with a preposition! How often did you hear this in school? I have good news: you can end a sentence any way you choose to. Ending sentences with prepositions is something I looked into. Thoroughly.
Let's define a preposition. It's a connective word that shows the relationship (in terms of time, space, cause, ownership, association, accompaniment, or manner) between a noun (or pronoun) and some other word in the sentence. Think "relationship," think "position," when you think "preposition."
Some of the most commonly used prepositions: about, above, across, after, against, along, amid, among, around, at, before, behind, below, beneath, beside, between, beyond, by, concerning, during, except, for, from, in, inside, into, instead, of, off, on, onto, out, over, past, pending, regarding, respecting, round, since, through, to, toward, under, until, unto, up, upon, with, within, without.
There are rules floating around--causing conflict and consternation--that were never really, truly, official grammar rules. They were often the personal preferences of people who liked to speak out on the subject. People in power. Like your fifth grade teacher or your great-aunt Matilda.
These good people are often the same ones who say (or said) we can never begin a sentence with "and," "but," "or," "also," or "however." But they're mistaken. In both cases, it's okay if it makes for an easy-to-understand sentence. However, make sure to use such words in very informal communications.
Sometimes using a preposition at the end of a sentence (terminal preposition) is awkward, and sometimes it's better to use one at the end. For example:
- Awkward: It is not easy to know that about which you are thinking.
- Natural: It's not easy to know what you're thinking about.
If good communication is your goal, just make sure that the sentence is clear for the reader or listener.