1.Robert is said to have studied abroad.

2.Her husband is known to have been writting the novel.

以上二个句子中Robert 和 Her husband 只是 is said 和 is known 语法上的主语，而不是意义上的主语，即is said 和 is known 描述的并不是 Robert 和 Her husband 这二个人，而是这二人作了什么事。所以谓语动词的语义是不完整的，需要补足成分来补足其意义的不完整，不定式短语对主语进一步描述，某人做了某事，到此谓语动词的意义才完整。所以不定式应该分析为主语补足语。但这二个句子并不是直接由主动句演变而来，因为say并没有 say sb to have done这种用法。它们是由下面句子演变而来的：

It is said that Robert has studied abroad.

It is known that her husband has been writing the novel.

is said 和 is known为主句的谓语动词，that从句为主语从句，是比主句第一个层次的结构。我们可以把that从句的主语（即低一级结构的主语）提到主句（高一级的结构）中作主语，把that从句的谓语动词改为不定式结构作主句的主语补足语，就得到你的二个句子：

Robert is said to have studied abroad.

Her husband is known to have been writting the novel.

在这二个句子中，主语都是从低一级的that从句提升到主句作主语的，主语只是语法上的主语，而不是语义上的主语，这种句子《剑桥语法》称为提升主语结构（raised subject）。