如题。

come along “和。。。（一道）来” ，后面通常有个with, 这时come 和 along 常被拆开。 例：
He came to our house last Friday along with his baby sister. 上周五他和他的小妹妹（幺妹妹）一块来我们家了。

come along “。。和。。（配搭）”（或译为：。。和。。是一套的），后面也通常要加个with，这是come 和along 常紧挨着。 例：
1。That blouse comes along with this vest. 那件女式上衣和这件背心是一套的。
2。 This offer comes along with the Plan B. “B计划”里才提供这些。（B计划里才含带这些）

happen “发生”，例：
1。The accident happened shortly after midnight. 刚过午夜意外就发生了。
2。It happens. 常有的事。

happen 表“碰巧”之意， 例：
Tom happens to be my highschool classmate. 汤母碰巧是我高中时的同学。
We didn't plan to meet them there. They just happened to be there when we got to the club. 我们也没特地计划安排和他们在那碰面，只是我们到了那家俱乐部时他们碰巧在那。