It is the third time that Ive explained the problem to you.

It is a long time since I saw Tom at my uncles.

It was fifteen years since she had left home. Her home village had greatly changed.


1.請問以上三個例句的it是形式主語還是強調？
2.我對以上句子的時態感到有不理解：
如句1，“It is the third time”是一般時態，為何從句卻是完成時（說話那一刻不正好是第三次嗎）？

句2句3結構一樣，可為何句2從句用過去時的saw，而句3從句卻用過去完成時had left？
而且她家鄉的巨大變化為何也是過去完成時啊？

It is the third time that Ive explained the problem to you.

說話時第三次講解已經發生過了。句子的邏輯意義等于我已經給你講解三次了。第幾次是從過去算到目前，所以與過去有關的目前要用目前完成時。

It is a long time since I saw Tom at my uncles.

主句的It is=It has been. 指從過去算到目前多久了。

It was fifteen years since she had left home. Her home village had greatly changed.

主句為一般過去時，從更早的過去算到隨后的一個過去時間有多長時間了。更早的過去要用過去完成時。家鄉的變化在主句時間之前，所以也要用過去完成時。

第三句改為下句也是正確的。

Ithad beenfifteen years since she left home. since的詞義決定了主從句謂語時間的先后，所以從句用一般過去年代替過去完成時。