Learn Finnish: Minun takanani seisoo mies, ja minä näen hänen olkapäänsä. - A man stands behind me, and I see his shoulder.

This is how postpositions work with personal pronouns! Because the man is behind the speaker, we use the genitive pronoun minun (my) and add the matching first-person possessive suffix -ni to the postposition takana (behind). Together, this makes minun takanani.

Could I drop the word minun and just say Takanani seisoo mies?

Yes! In standard written Finnish, first-person and second-person pronouns like minun can be dropped because the suffix -ni already tells us who it belongs to. However, you must always keep the pronoun for third-person words, like hänen olkapäänsä.

Why does the sentence say Minun takanani seisoo mies instead of Mies seisoo minun takanani?

Finnish often starts a sentence with a location to introduce a new situation, similar to saying 'There is a man standing behind me' in English. Putting mies (a man) at the end shows that he is a new, unspecified person being introduced to the listener.

The shoulder (olkapää) is the object of näen (I see). Shouldn't it have the genitive -n ending?