reason用于事情可指“理由，原因，道理，情理”，用于人可指“理智，理性，思维能力，推理能力”。reason后常可接动词不定式或for引导的短语或从句作定语，引导从句的why在口语中常省略。reason作主语时，可接that引导的从句作表语。

reason的用法

首先强调一下，reason后why引导的不是同位语从句，而是定语从句。

1)reason+定语从句

a.关系副词充当定语从句的原因状语，可以用why或that或省略。

The reason I called was to ask about the plans for Saturday.

备注：关系副词可以用for which代替。

b.关系代词充当定语从句的主语或宾语，可以用that或which，与一般的定语从句用法相同。

What is the reason that he gave for this?

What was the reason which led the slaves to leave the estates in British Guiana?

2)reason+同位语从句，用that引导。

We decided against installing new heating apparatus,for the reason that it would be too costly.