请问形式主语It后面是否可以直接用“It is to do sth”?

请问形式主语It后面是否可以直接用“It is to do sth”? 形式主语 it should be avoided to do sth. it作形式主语的问题it's +adj+to do sth是否等于it's+adj+doing sth例如it's wr It is time for sb to do sth 存在形式主语吗? it is one's duty to do sth.其中______是形式主语,真正的主语是______. It is +adj+ to do sth中It是形式主语吗?真正的主语是动词不定式to do st be+adj+to do sth是不是都用it做形式主语? It is adj.for sb.to do sth.和It is adj.of sb.to do sth.改成实际主语 请问,It is up to sb.to do sth. It's +形容词+(for sb.)+to do sth .什么为形式主语? it takes sb some time to do sth是形式主语吗 请问It seems to do sth.可以转换成It doesn't seem to do sth还是It seem