[Columbine]

I know most text tends to use plain form, MMM, but the OP used 'masen' and the question was about the use 'ja' and 'dewa'.

I'll rephrase it then; I was taught that 'ja' is more usually spoken and less usually written and 'dewa' is the reverse.

If that's still wrong, then please let me know so I can add another complaint to the 'why my teacher needs to be fired' list.