While I was talking about the difference between two people or things, I tried to shorten the contrast by using not and don't and to this moment I don't know whether what I do is grammatically correct or not?
For example:
Some people think taking part in those classes could be useful and some others don't.
I like watching movie,whereas my sister doesn't.
They are good at selling but another group not.
some people can afford that house.others can't.