What seems to us as bitter trials are often blessings in disguise.

把介词短语 to us 拿出来。看看句子结构，便一清二楚。

What seems as bitter trials are often blessings in disguise.（？）

What seems like bitter trials are often blessings in disguise.（正）

What seems to be bitter trials are often blessings in disguise.（正）

What seems bitter trials are often blessings in disguise.（正）

What 是“融合性关系代词”，等于说：The thing/Anything that...

What seems to us as bitter trials are often blessings in disguise.

这个句子不够规范，有二处语法错误。

1 seems 应为复数seem.

2 介词as应为介词like或直接删去，或改为to be.

What seem to us bitter trials are often blessings in disguise.

What seem to us like bitter trials are often blessings in disguise.

What seem to us to be bitter trials are often blessings in disguise.

人或事物作主语，seem后接形容词、名词、不定式、like介词短语，as if/as though, 但不接as介词短语。

what引导的名词性关系分句作主语，what本身是从句的主语，此时主句和从句的谓语的数应该一致。因为what = the thing(s) which.

以下为BNC和COCA语料库有关例句，说明主句和从句谓语的数一致。

In Nigeria and elsewhere, what seem to be religious grievances almost always have secular causes underlying them. 

What seem to be four candlesticks -- designed in the same sugary style -- stand guard around it and a fifth, a little distance from the main confectionary, is crowned with a gilded figure.

What seem tobe the earliest human figures on Geometric vases are less precisely geometrised than these.

What seems to be required now is something more in the nature of a decision. 

What seems to be driving them is a desire to create the small-town feeling that is fast disappearing in America.

What seems tobe needed is an approach which recognizes the necessary contrivance of pedagogy and seeks to guide learners through graded negotiating tasks. 

What seems to us as bitter trials are often blessings in disguise.

= What seems as bitter trials to us are often blessings in disguise.

=  What seems to be bitter trials to us are often blessings in disguise.