Written french is largely codified by the french academy which is very conservative (so the written language did not evolve much in the past 100 years) whereas

Written french is largely codified by the french academy which is very conservative (so the written language did not evolve much in the past 100 years) whereas the spoken language evolved independently. It's a bit like slangs in English if you want but to a whole new level, the two parts of the language don't have much in common nowadays. Tenses, pronouns, grammar, sentence construction and words are different.