I had not been reading for half an hour____I heard steps out

状语从句,应填when,作为主句动作发生的背景 when引导的从句包含的是新的或主要的信息, 主句包含的则是已知或次要的信息.此时,从句总是位于主句之后,其谓语动词为一般过去 时. 例句： I was just getting into the bath when the telephone rang;
I was about to ask after his wife when I suddenly remembered that they were getting a divorce.
I had not been reading for half an hour when I heard steps outside