Translation:This is the guard whom I talked to.
It is not correct! Ο φύλακας στον οποίο μίλησα or ο φύλακας για τον οποίο μίλησα, ο φύλακας με τον οποίο μίλησα are possible alternatives, with different meaning each one but not ο φύλακας τον οποίον μίλησα. I am native speaker. Please correct it!
I came across to this phrase again after one year. It is wrong. Please correct it. THERE IS NOT SUCH a phrase in Greek!
Why do I have to say "This ist the guard..."? Why is "He is the guard..." not correct?
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 :-)