两句中皆没有形式主语,Two-thirds of the students are girls主语是Two-thirds of the students 而Whether she will join us is still a question.主语是Whether she will join us这个从句.
形式主语是指主语部分相对于句子的其他成分太长,若按主谓宾顺序有点头重尾轻,不顺眼,所以通常用it 作为形式主语,而真正的主语后置.
如 Whether she will join us is still a question.就可以有形式主语,就是It is still a question whether she will join us .