9 all at once; all of a sudden; in a flash

● all at once是介绍词组，在句中常作状语，位置灵活，在句首时可不用逗号隔开。如：All at once we heard a loud noise.（突然，我们听见一声巨响。）All at once Christine felt herself turn a little giddy, and fell down. A stranger sent her to a hospital.（突然克丽丝蒂娜觉得自己有点头晕目眩，并跌倒在地。一位陌生人送她到了医院。）While we were talking, all at once Mary screamed.（我们在谈话时，玛丽突然尖叫起来。）The storm broke out all at once.（暴风雨突然来临。）

● all of a sudden的意义与用法同all at once，可换用，只是all of a sudden更突然。词组位置灵活，放句首时可用逗号隔开。如：All of a sudden, I remembered that I had forgotten my purse.（我突然想起我已忘记带我的皮夹子。）All of a sudden a man burst out of the hedge, and made off down the road.（突然有个人从篱笆里闯了出来，从公路上跑掉了。）We heard a tremendous explosion all of a sudden.（我们突然听到一声剧烈的爆炸。）As she gazed, all of a sudden the light went out.（当她仔细看时，突然，灯灭了。）

● in a flash本意“立刻” “马上”，可引用为“突然地”，是介词词组，作状语，位置灵活。如：In a flash the boy got over the fence and started running down the road.（瞬间，那个男孩越过篱笆开始向路上奔去。）He was looking over his notes for English class and in a flash he knew what he would write his paper about.（他翻阅英文课笔记时，突然知道要写些什么了。）I didn’t see her meaning at first, then it all came to me in a flash.（开始我没明白她的意思，后来我却悄然大悟。）We were watching the bird eat the crumbs; then I sneezed and he was gone in a flash.（我们在看小鸟吃面包屑，这时，我打了个喷嚏，那小鸟突然飞走了。）