It seems that从句与sb/sth seems to do之间有转换关系。《剑桥语法》称后者为提升主语结构（raised subject)。即把that从句的主语提升为主句的主语，that从句的谓语改为不定式结构。当that从句谓语为系表结构且表语为归属性表语时，to be可以省略。例如：

It seems that Tom likes swimming. 从句谓语为静态动词

Tom seems to like swimming. 不定式用一般体

It seems that Tom goes to work by bus every day. 从句谓语为习惯意义

Tom seems to go to work by bus every day. 不定式用一般体

It seems that Tom is reading a book. 从句为进行时态。

Tom seems to be reading a book. 不定式用进行体，to be不能省略

It seems that Tom have left. 从句谓语为完成时态

Tom seems to have left. 不定式用完成体

It seems that Tom left yesterday. 从句谓语为过去时态

Tom seems to have left yesterday. 不定式用完成体

It seems that Tom is an engineer. 从句谓语为系表结构，表语为归属性

Tom seems (to be) an engineer. 不定式to be可以省略

It seems that Tom is the person you are looking for. 从句谓语为系表结构，表语为确认性

Tom seems to be the person you are looking for. 不定式to be不可省略

It seems that Tom is angry. 从句为系表结构，归属性表语

Tom seems (to be) angry. 不定式to be可以省略

It seems that Tom is excited. 从句谓语为系表结构，归属性表语

Tom seems (to be) excited. 不定式to be可以省略

It seems that the book is interesting.从句谓语为系表结构, 归属性表语。

The book seems (to be) interesting. 不定式to be可以省略

It seems that Tom is respected by all. 从句谓语为被动语态

Tom seems to be respected by all. 不定式to be不可省略