"The same ship is at port."
Translation:La stessa nave è nel porto.
So, I'm guessing I got this wrong because when the adjective comes after the noun, that means it is describing it. Right? Therefore, I put, "La nave stessa e' al porto" and it was considered wrong. But, could anyone verify this?
I wonder what is the difference between "barca" and "nave"? Size of the vessel?
it is the same difference existing between boat and ship. But you are right nave is in general larger in size.
It would be "uguale". But I think this only means the same in the sense of 'equal', not the sense of 'identical', so it isn't right in this context.
I'm not a native English speaker. Could someone explain why is there no article before "port" in the English version? Or how is this different from saying "at the port"? Just "at port" sounds very unusual to me.
"In port" is the common English idiom. It is a contraction of "in the port", but it is a bit more than just a location. It also indicates the ship's current status. Interestingly, the opposite status is "at sea". Anyone who knows about maritime transport will prefer "in port". Anyone who doesn't may well use a longer phrase - or, like Duolingo, a wrong one.
A related status is "in dock", which is like in port but implies that the boat is being un/loaded or repaired. "In dock" is sometimes applied as an idiom to other equipment being repaired: "my car (or computer, etc.) is in dock". Britain's naval history has given English a very large number of idioms and sayings like this. See for example http://www.plymouth.gov.uk/homepage/leisureandtourism/libraries/history/navalhistory/navalsayings/navalsayingsac.htm