be doing/ be about to do/ be on the point of doing/ had done
, when
(when: 这时,强调一个动作的突然发生)1) I was walking along the river, when I heard a drowning boy cry for help
2) I was about to leave when it began to rain
3) I had just finished my test paper when the bell rang, announcing the exam was over
It was (not) +时间段 +before +一般过去时 "过了一段时间就……"It will (not) be +时间段 +before +一般现在时 "要过一段时间才会……"It is/ has been +时间段 +since
It was +点时间 +when
It was +时间状语 +that
(强调句)1) It was not long before he sensed the danger of the position
不久他就意识到他处境危险
(动作已发生)2) It will be half a year before you graduate from the school
还有半年你才从这个学校毕业
(动作未发生)3) It is 3 years since he worked here
= he left here
(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) It was 3 o'clock when they received the telephone