In general they are not interchangeable. "shang wang" (上网）is used as a verb, it means "surf the internet". For instance: what are you doing now? To answer that you are online (surfing) you can say: wo zai shang wang. "wang shang "(网上）describes the status of being online. All your examples should use this word: wo zai wang shang mai ji piao <-> I buy plane tickets online. wo zai wang shang xue zhong wen <-> I go online to study Mandarin. zhe bu dian yin wang shang mei you <-> The movie isn't available online. There are also other ways to say that you are online. Ta xian zai bu zai xian<-> he/she is currently not online. It also describes the status of being online. Usually "zai xian" is used to describe that a person is online. Also "wang shang" can be used together with other words to create a new word. e.g. wang shang yin hang <-> online banking

明年夏天 is how we usually say it. But in other cases like next Sunday we say 下个星期天

明年夏天 is how we usually say it. But in other cases like next Sunday we say 下个星期天

大家好！ 请你看一下我写的课文。

我的家庭不那么大，一共有四口人。家人们的性格都不一样。比方说，我的哥哥很厉害。他又活泼又幽默，他常常讲笑话，大家都哈哈地笑！但是他有一些懒。尽管他的经理特别严格，我哥哥还是马马虎虎地工作。因为他不努力工作，所以他很轻松，不那么忙。我妈妈是个能吃苦的人。她每天努力工作，遇到困难的时候很冷静，我以我妈妈为傲。虽然她的个子有一点矮，不过她特别漂亮，尤其是她的头发。我奶奶是个很热情的人。我有什么问题的时候，就问问她。她还特别照顾我们，每周末都做好吃的菜。她还有别的优点，她很有耐心。我跟我哥哥差不多，是个有一点懒的人，有的时候不想做课外工作。