"of"在句首的原因及用法如下：

强调

"of"放在句首通常是为了强调某个特定的名词或短语。例如，在句子“Of all his daughters, he loves the youngest named Nana most.”中，“of all”强调了“所有女儿”的范围，使得句子更具强调效果和平衡感。

平衡句子结构

当"of"短语修饰的主语较长时，将其置于句首可以使句子结构更加平衡和匀称。例如，在句子“Of the three brothers, he is the most hardworking.”中，“of the three”作为定语修饰“brothers”，放在句首使得句子更加流畅。

表示范围

"of"在句首还可以表示范围，通常用于表示某个动作或状态发生的范围。例如，“Of the five people, only two are coming to the party.”（五个人中只有两个人来参加聚会）。

示例

强调

Of course, he is the best choice.

Of all the books, she likes the one written by her grandfather the most.

平衡句子结构

Of the team, she is the most experienced player.

Of the students, he runs the fastest.

表示范围

Of the cities, New York is the largest.

Of the students, those who study hard are the most successful.

总结

"of"在句首主要用于强调特定内容、平衡句子结构或表示范围。具体用法需要根据句子的语境和结构来判断。