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.请问以上三个例句的it是形式主语还是强调？
2.我对以上句子的时态感到有不理解：
如句1，“It is the third time”是一般时态，为何从句却是完成时（说话那一刻不正好是第三次吗）？
句2句3结构一样，可为何句2从句用过去时的saw，而句3从句却用过去完成时had left？
而且她家乡的巨大变化为何也是过去完成时啊？
It is the third time that Ive explained the problem to you.
说话时第三次讲解已经发生过了。句子的逻辑意义等于我已经给你讲解三次了。第几次是从过去算到目前，所以与过去有关的目前要用目前完成时。
It is a long time since I saw Tom at my uncles.
主句的It is=It has been. 指从过去算到目前多久了。
It was fifteen years since she had left home. Her home village had greatly changed.
主句为一般过去时，从更早的过去算到随后的一个过去时间有多长时间了。更早的过去要用过去完成时。家乡的变化在主句时间之前，所以也要用过去完成时。
第三句改为下句也是正确的。
Ithad beenfifteen years since she left home. since的词义决定了主从句谓语时间的先后，所以从句用一般过去年代替过去完成时。