1. be about to do when 当正要做什么的时候（突然）怎么样了
e.g. I was about to leave the room when he came in.我正要离开房间时（突然）他进来了。
2. doing when 当正在做什么的时候（突然）怎么样了
e.g. She was thinking of her friend when Tom called her.她正在想她朋友的时候Tom（突然）叫了她一声。
3. had done when 刚做完什么的时候（突然）怎么样了
e.g. I had just walked out of the kitchen when someone knocked at the door.我刚从厨房出来，（突然）有人敲门。