Complex sentence: Nouns may be further divided into declensional subtypes, type-I and type-II, with the basic difference being that the former has characteristic terminations in the direct singular while the latter does not.
Split into simpler sentences: Nouns may be further divided into two classes based on declension, type-I (marked) and type-II (unmarked). The basic difference between the two categories is that the former has characteristic terminations in the direct singular while the latter does not.