My poor students! In tonight's class we're introducing additional nouns of the second declension, which means that the class will have to learn the gender of the noun along with the noun itself. That's just the nature of the beast when it comes to languages like Greek, Latin, and German. As for German, I've found the following mnemonic of suffixes helpful:
Der --
- ig
- ling
- ismus
Die --
- keit
- heit
- ling
- ei
- shaft
- ität
Das --
- tum
- un
- chen
- ment
- lein
In Greek, -os usually works for masculine nouns, and -on for the neuters.
So much to know!