6、等。2.since+过去一个时间点;since+时间段+ago;since+从句(过去时);Itis+时间段+since+从句(过去时);3.全优课堂P99短暂性动词转换延续性动词都有哪些?1.Both his parents look sad.Maybethey ________what's happened to him.A. knewB. haveknownC. mustknowD.willknow2.He has _____been to Shanghai,has he?A. alreadyB.neverC.everD.sti