It (This, This evening, yesterday...) is (was, will be) first (second, third...) time (day, month...)...结构中的从句，要求用完成时态。
【例如】
Is this the first time you ve been to Beijing?This is the eighth month that I have been out of work.This is the second time that the goods produced by our factory have been shown in the International Exhibition.在no sooner…than, hardly/barely/scarcely…when,等的句型中，主句要用过去完成时。
【例如】
No sooner had he seen me than he left the room.Hardly had the helicopter landed when the waiting crowd ran toward it.Scarcely had I seen the lightning when I heard a clap of thunder.