What's the difference between 'ne' and 'ye'?
예 ( ye ) is formal while 네 ( ne ) is simply standard. 응 / 어 ( eung / eo ) is informal.
You must have heard that Koreans do not use annyeong or annyeonghaseyo instead they use wat seo or wat ni or wat nya is that right?
Wow where did you hear that? Lol
When someone comes and if they are really familiar to you(such as your friends), you can say 왔어?, 왔니?, 왔냐?
But they are informal way so that you should be careful for using them.