bedoing/beabouttodo/beonthepointofdoing/haddone…,when…(when:这时,强调一个动作的突然发生)1)Iwaswalkingalongtheriver,whenIheardadrowningboycryforhelp
2)Iwasabouttoleavewhenitbegantorain
3)Ihadjustfinishedmytestpaperwhenthebellrang,announcingtheexamwasover
Itwas(not)+时间段+before+一般过去时“过了一段时间就……”Itwill(not)be+时间段+before+一般现在时“要过一段时间才会……”Itis/hasbeen+时间段+since…Itwas+点时间+when…Itwas+时间状语+that…(强调句)1)Itwasnotlongbeforehesensedthedangeroftheposition
不久他就意识到他处境危险
(动作已发生)2)Itwillbehalfayearbeforeyougraduatefromtheschool
还有半年你才从这个学校毕业
(动作未发生)3)Itis3yearssinceheworkedhere
=helefthere
(since从句中的谓语动词若是延续性动词,要从这个动作结束的时候算起)4)Itwas3o’clockwhentheyreceivedthetelephone
5)Itwasat3o’clockthattheyreceivedthetelephone
once…一旦……,表示时间和条件1)Onceyoustart,youwillnevergiveup
2)Onceyouunderstandwhattheteacherexplained,youwillhav