I am very confused on when to use an, in and auf.
I am using the "Teach yourself complete German" book. In unit 20, in the practicing section, they made an exercise where I must fill the gaps.
It is like this: "..... Nein, wir haben Urlaub ... der Ostsee gemacht, ... der Insel Rügen".
My answer was "..... Nein, wir haben Urlaub in der Ostsee gemacht, in der Insel Rügen". But in the Solutions section they wrote "..... Nein, wir haben Urlaub an der Ostsee gemacht, auf Insel Rüge". Why?
An means near it and therefore it is the appropriate preposition for Ostsee, since you probably spent not the whole time in the water, which in der Ostsee would imply.
(I admit, that this is tricky, since Rügen as an isle actually is in der Ostsee. For a huge unspecified region as Südsee however, in may still be appropriate.)
For islands in general auf is the preposition to use, while towns, regions and countries would require in. Auf is further used for conventions, parties etc. I'm afraid, there are just a few rules of thumb, what to use when, see e.g. lokale Präpositionen under the section Präpositionen, die auf die Frage "wo" eine Antwort geben können.
(See also this related question.)
