it is +adj + that +从句 等于什么
it is + adj + that + 从句 = That 从句 is adj.
比如it is necessary that we learn english.=That we learn english is necessary.
在这个从句明漏中that从句做主语，典型的名词性从激漏烂句做主搜穗语，that是个conj.
it is +adj + for sb to do sth.
it is necessary that we learn english.
it is necessary for us to learn english.
等于it is +adj+for sb to do sth