(Should be coded in separately in relevant games)
Different forms:
в - во: Second form is used with consonant clusters starting with в, ф, мн. See if this should have its own ru_class letter.
о - об - обо: Second form used with words starting in non-iotated vowels. Third form with consonant clusters as ditto.
Different praepositions:
в/на/к: к triggers use of dative, other two of accusative. к is used with animate nouns (I am going to the Celts), в with most other countries (I am going to France), на with islands and peninsulas (I am going to Cuba). Yet to know how to trigger the use of dative.
из/с: all use genitive. из corresponds to both к and в (I come from France/from the Celts), из to на (I come from Cuba)
(Should be coded in separately in relevant games) Different forms:
Different praepositions: