重点讲解：
1. come upon
偶然碰到（或发现）；
eg. I've just come upon a beautiful poem in the book.
我在这本书里偶然发现了一首优美的诗。
eg. If you're unlucky enough to come upon one of these dangerous creatures. The safest thing to do. . . Is run the other way.
如果你非常不幸地遇到一个这样的危险生物，最安全的做法，就是择路而逃。
2. all of a sudden
突然地；出乎意料地；
eg. All of a sudden he turned about and saw me.
他突然转过身来看见了我。
eg. It became overcast all of a sudden.
天一下阴了下来。
3. face to face with sb.
面对面；直面；
eg. We were strolling into the town when we came face to face with Jacques DuBois.
我们信步向镇子走去，迎面碰上雅克·杜波依斯。
eg. I found myself face to face with her in the market, so I couldn't get away.
我在市场上和她面对面碰上了，所以脱不开身了。
4. at that instant
在那个时候；在那一瞬间；
eg. At that instant the museum was plunged into total darkness.
就在那时，博物馆陷入了一片漆黑。
eg. At that instant, I heard the sound of steps, followed by abrupt knocks on my door.
就在此时，我听见楼梯响，接着是急促的敲门声。
5. be about to do sth.
即将（做…）的；快要（发生…）的；
eg. He was about to speak. Again he bottled himself up.
他想说话又忍住了。
eg. I was just about to call you.
我正准备打电话给你。