be+adj+to do和be+adj+that从句，这种句式To do sth和that 从句 是表语吗

这个结构应该是
It
+
be
+
adj.
+
to
do
sth.或
It
be+adj+that
从句
1，
it
是形式主语，to
do
sth
是真正的主语
句子这种写法是为了避免结构的复杂，如同我们汉语常说的头重脚轻一样。
2,
同样，that
引导的是一个主语从句,因为它代表的是开头的形式主语"It"所指的真正内容.
3，某些It
is
adj
to
do
sth
的句子可以转换成
To
do
sth
is...
或者that从句
因为本质是一样的。
4,
其中的形容词(adj)后面还可以加for/of
sb
:
It
is
adj
for/of
sb
to
do
sth
当形容词是形容一个人的品质时用of
当形容词是对于一个人来说会怎么样时用for
It's
so
kind
of
her
to
help
me.
It's
too
heavy
for
her
to
carry.

扩展阅读：be comfortable doing ... catch sb doing sth ... see sth doing sth ... be adj to do sth ... be adj for sb to do ... is being adj ... have been doing ... be doing when ... look forward to ...