that引导的定语从句放在句首时，主要充当主语或宾语成分，具体取决于先行词和从句结构。以下是详细说明：

一、that作主语的情况

位置要求

当定语从句位于主句之后时，that可作主语。

例如：

*The book that I borrowed is very interesting.*

*He is the man that helped me.*

省略现象

当先行词被the only/second only等限定时，that可省略。

例如：

*This is the house that we lived in.*

二、that作宾语的情况

直接宾语

that可代替先行词作动词或介词的宾语，且通常可省略。

例如：

*She likes the movie that was released last year.*

*He passed the exam to which everyone was expecting.*

介词宾语

在介词后引导定语从句时，that不可省略。

例如：

*This is the letter to which I sent you.*

三、注意事项

先行词为物时：

只能用that或which引导定语从句，且that不可省略。

先行词为人时：可用that或who引导，但whom作宾语时可省略。

关系副词：如where、when、why等，引导定语从句时作状语，与that功能不同。

四、错误用法示例

that作表语：

*He is the man that he likes.*（错误）应为 *He is the man whom he likes.*

that引导非定语从句：

*That made the movie interesting.*（错误）应为 *Which made the movie interesting.*

综上，that引导的定语从句句首主要作主语或宾语，需根据语境选择合适的关系词，并注意省略规则。