A port is a city with a harbour, which is an area where ships can moor, typically consisting of a sheltered bay with some jetties, docks, and similar seafaring facilities.
I presume these are both hamn in Swedish, since the word is thus glossed on this course. The same applies to many other languages.
Ports are not only cities with a harbour, but can also be "commercial places along the coastline that are used for import and export of goods and cargo from one country to another. One can relate a port with an airport where airplanes arrive and depart", whereas "a harbour can be a man made or a natural feature connecting a piece of land with a large water body that is mainly used to provide shelter to ships and vessels from bad weather. Harbours are used for safe anchorage of ships. For differentiation of "port" and "harbour" in English, see http://www.differencebetween.com/difference-between-harbour-and-vs-port/.
Okay, I have just difficulties to remember the difference between torg and marknad, because the finnish word "tori" (pronounced the same way as torg) means both square and marketplace. I link "tori" more strogly to marketplace (marknad), than square (torg), so they get easily mixed up.
I have thought that a lot and I think that the marketplace should accept as a secondary alternative. The English linguists let some countries use the translation marketplace and they have inspected the schoolbooks too. Why do not they require to use the word square? I hope that someone explains to me why it is so wrong.