Complex sentence: The subdivision is based solely on the use of the "s" for pluralization and the weakening of some consonants, but that makes the categorization highly problematic since there is much higher lexical similarity between all dialects of Italian and French than between French and Spanish.
Split into simpler sentences: The subdivision is based solely on the use of the "s" for pluralization and the weakening of some consonants. This makes the categorization highly problematic since there is much higher lexical similarity between all dialects of Italian and French than between French and Spanish.