What's happening? What's up? 这两个一般可以当做打招呼,不过我们中国也有很多人习惯第一句就来：你在干嘛What's happening? 是很久不见才用,和How are you意思也差不多. What's going on?和What's happening?又差不多,但一般在邮箱,电话里用.