说for sb.to do sth.时,只能用在形式主语It开头的句子里,比如：It must have been impossbile for the staff to say "it serves him right."在It is ...for sb.to do sth.的结构里,sb.是逻辑上的真正主语.
而The staff must have found it impossible not to be tempted to say "it serves him right"这句话里,已经有了真正的主语staff,不能再用for引出逻辑的主语them,否则就会出现主语的重复.