How long does it take for a letter to get to America from Be

How long does it take for a letter to get to America from Beijing?
it takes seven days for a letter to get to America from Beijing.
两个句子中的it都是形式主语,真正的主语是动词不定式,而动词不定式有自己的逻辑主语（a letter）,即构成不定式的复合结构.而不定式的逻辑主语必须靠介词（通常是of /for)来连接.此处for a letter to get to America from Beijing正是不定式的复合结构.
It is very kind of you to say so.
It is very important for us to learn English well.
上两句中的介词都不能省.