1,嗨 is something like Hi ,a very casual greeting! 你好 will be a formal one,usually using at the first you meet someone,or the persons you say to are someone like you teachers 、your boss 2,变成 is the word like to become into ,for example,we would say "打了架后，他们变成了朋友（they became friends after a fight"; 改 is like “ to correct" or "to change(a schedule),for example: 这个字你写错了，要改一下；今天我没有时间，可以改明天吗？ 3，上网 is a verb,something you do by internet, for example,我上网看电视剧；”网上“is like ”a place",for example, 我在网上看到这个新闻的（I saw this news on the internet".

1. 嗨is informal and 你好 is formal. 2. 变成means sth change into other thing is different from before. 例，她变成了一个小气的人。改means correct the fault or change a schedule...such as改错，改天。 3. 上网，sb does sth in the Internet。网上，online.