Translation:We thought Saint Petersburg was a city in Germany.
Good old Peter the Great for dragging Russia into the present. Bearded men: watch out.
Actually the suffix "bourg" (Strasbourg, Cherbourg) is of latin origin. This suffix has the meaning "town". "Salzburg", on the other hand, has the suffix "burg" which means "mountain" in german.
Burg doesn't mean mountain. That would be Berg. Burg means castle as in medieval fortress where e.g. knights lived. Source: I'm a native speaker
You can drop «что» in such sentences. But of course, if you use it, it’s OK too.
In another course I was told that что is required in Russian even when it isn't in English. Are there some times when it's necessary to say что and others where it isn't?
I think it depends on the verb. With some verbs «что» can be easily dropped (like думать 'think', полагать 'to believe', слышать 'to hear', видеть 'to see' when it is used metaphorically), while with others it sounds colloquial and informal (видеть 'to see' when it refers to actually seeing something with your eyes, знать 'to know').
Because of the dash after petersburg. That dash serves as "is" in English. City of petersburg would've been город петербурге(petersburg in it's genitive case)