关系代词 `that` 在定语从句中可以充当主语，其例子和意思如下：

例子:

"The book that I bought yesterday is very interesting."

"The man who is standing there is my teacher."

"He asked about the factories and workers that he had visited."

"They are talking about the people and things that we have visited in this/the country."

"This is the highest score that I can get."

"She is the girl that talked to you yesterday."

"The coat （that） I put on the desk is blue."

意思:

`that` 作为关系代词，指代先行词（被定语从句所修饰的名词或代词），并在定语从句中充当主语或宾语等语法成分。它可以指人也可以指物，并且在从句中的成分根据先行词和从句的结构而定。

建议

在使用 `that` 作为关系代词时，需要注意它所指代的先行词在从句中需要充当的成分（如主语、宾语等）。

`that` 在引导定语从句时，通常不能省略，但在某些情况下（如从句作为宾语），可以省略。

在先行词被某些限定词（如 `all`, `any`, `no` 等）修饰时，通常使用 `that` 。