It seems that these grammar is easy to me.

这个句子可以转换句型。即用that从句的主语作seem的主语，that从句的谓语动词改为不定式，与seem构成复合谓语，得到下句。

These grammar rules seem to be easy to me.

当seem后的不定式为系词 to be, 表语为归属性表语时，to be 可以省略（也可以不省略）。

These grammar rules seem （to be） easy to me.

表语为确认性表语时，to be不可省略。例如：

It seems that Tom is the boy who broke the window.

Tom seems to be the boy who broke the window.

Tom seems the boy who broke the window. (错误。to be不可省略）