For example:
Mi ne povas decidi kio(n) paroli pri. (not sure about the accusative)
Mi ne povas decidi kion priparoli.
I've said something similar to #1 before not knowing whether it was okay or not. Someone pointed out to me that it probably would be better to say #2, but he wasn't completely sure either, as we are both newer Esperantists.
So, are either of these even correct? Is it wrong to end a sentence with the word pri? Is there a better way to say something like this?
That's a clear anglicism. Some people from scandinavian countries also sometimes speak like that, because their languages also know these grammar structures. But in Esperanto it is clearly wrong. A preposition (PMEG terminology "rolvorteto") stands before the noun or pronoun.
So the correct sentence is:
Mi ne povas decidi pri kio paroli.
Alternatively you can use the preposition as a prefix, which is inparticularly popular for pri:
Mi ne povas decidi kion priparoli.
No. 1 is incorrect. No. 2 is okay but evades the problem by using a different verb.
Your intention is probably:
Mi ne povas decidi, pri kio paroli.
A preposition cannot appear at the end of a sentence (except of course for sentence fragments like Ĝis!). This can sometimes be fixed by adding -e to make an adverb:
Mi parolis anstataŭe.
In your sentence, the thing you are deciding must appear straight after decidi, but without severing the connection to the verb; I added a comma to make it clearer. And of course if you don't like how it sounds, you could just say something like:
Mi ne povas decidi la temon de mia parolado.
My opinion is that this is an English grammar rule in an Esperanto sentence:
I cannot decide what to talk about.
As far as I know, that is not correct in Esperanto (please correct me if I am wrong). I think prepositions should go with the thing they refer to. I would use either option 2 or (maybe because of my native Bulgarian influence):
Mi ne povas decidi pri kio paroli.
Remember that prepositions are, by definition, in the pre-position. That means they need to come before a word. (English bends that rule a lot, but Esperanto does not.)
It's not just pri, but this is true for all Esperanto prepositions. They should never be used at the end of a sentence.
Here is a list of prepositions and other particles that you should never find at the end of a sentence.
pri, super, trans, post, kontraŭ, de, inter, ĉirkaŭ, ĉe, antaŭ, post, sub, apud, preter, per, malgraŭ, dum, krom, for, por, pro, el, en, sur, ekster, super, per, po, ol, tra, ĝis, al, sub, kun, sen, laŭ, krom, anstataŭ, ekster, malgraŭ, da
The only possible exception is da - at that only in informal spoken Esperanto when there are no children or vulnerable adults within earshot.
