不一定，还可以用it was... since ＋had done。如：It was years since I had seen her. 在那之前我已经有好久没有见到她了。
从句谓语用一般过去时，这些动词都是终止性动词。It has been three years since he left.
是的，有it也有since,since后跟一般过去式，语态就只能是现在完成时，且it is可以代替现完。
