状语从句

再看看同样使用频率较高的状语从句：For example, if sufficient sky trains and underground train systems were built and effectively maintained in our major cities, then traffic on the roads would be dramatically reduced.句中，if 引导的是条件状语从句：if sufficient sky trains and underground train systems were built and effectively maintained in our major cities. 主句一般而言表示基于此条件的结果。我们试着来总结一下它的使用公式：if+句子(条件)+ then+句子(结果)。需要注意的是由于是虚拟条件，if 引导的条件从句用一般过去时，主句用虚拟语气较为常见。(具体请参照例句理解)

宾语从句

They may think that it is better to continue in a particular job, or to do something completely different from a university course.句中，that 引导的从句：that it is better to continue in a particular job, or to do something completely different from a university course. 作think 的宾语。That 本身在从句中不做任何成分。我们来探讨宾语从句的使用规律：主语+谓语+that+完整句子。就构成了一个宾语从句。