Translation:This is the guard whom I talked to.
You have a good question there. Take a look at this comment by Juan_Humaran and the answers in the discussion of the reverse sentence: https://www.duolingo.com/comment/17881917$comment_id=17881918 There the Greek sentence was changed accordingly, this one here is locked and will have to wait for the revised tree, I guess.
EDIT 11/1/2017: Or just see Stergi3's comment on this thread :-)
I have come to bring good tidings. There is no such rule in the English language. I've researched this thoroughly and you can too if you doubt it.
This is attributed to Winston Churchill but probably is not his, "This is the sort of nonsense up with which I will not put!
So, relax and use that preposition in comfort. I could bring in a few other common mistaken "rules" such as you cannot start a sentence with a conjunction. But actually you can. Or never split an infinitive but we say to boldly go where others have feared to tread.
Thank you for your interest and we hope this has been helpful.