move or turn back; retreat 後退; 撤退; 退却
*The enemy fell back as our troops advanced. 我军前进, 敌军後退. fall back on sb/sth (be able to) go to sb for support or use sth when in difficulty 有困难时（能）求助于某人, 依靠某事物
*At least we can fall back on candles if the electricity fails. 停电时我们至少可以使用蜡烛.
* She's completely homeless at least I have my parents to fall back on. 她是真正无家可归--我至少有父母可依靠.
1. Everyone knows that if he shouts in the vicinity of a wall or a mountainside, an echo will come back. 大家都知道，在墙壁或山腰附近发出的喊声，就会听到回声。
2. Before returning home at night, he took a shower and changed back into his suit. 晚上回家前,他洗个淋浴,又换上他那套西装.
3. Because of this, we kept looking back, wondering if we were leaving a trail of oil and petrol behind us. 因此，我们不时地掉过头，怀疑车后是否留下了机油和汽油的痕迹。
4. Before we had time to worry about what might happen, we were back on the plain again. 我们还未来得及担心后果，车已重新开上了平地。
5. Before returning home at night, he took a shower and changed back into his suit. 晚上回家前，他洗个淋浴，重新换上那身黑色西服。
6. By January 10th, I was back to where I had started from. 到了1月10日，我恢复了原来的作息时间。
7. Back in the train, we told each other what a wonderful time we had had that day. 回去的火车上，大家都说，这一天过得真愉快。
8. But I am going to buy them all back again now!" 不过现在我要把这些东西一一买回来。”
9. Back in the early 1960s, when the laser was first being developed, it was viewed by some as a fascinating research tool, others called it a science-fiction toy. 追溯到二十世纪六十年代初期,当激光最初被开发利用时，一些人视之为神奇的科研工具，另一些人则称之为科幻小说里的玩具。
10. But hardly had he eaten the last mouthful when he was back in his study again. 但几乎等不及吃完最后一口，就又回到了他的书房。