1. 语法讲解：
条件状语从句
从句的内容作条件状语。
常用引导词有：if, unless, suppose, supposing, provided that, in case, so long as, as long as, on conditionthat等。
As long as you give all of your money to us, we will let you off.
只要你把你所有的钱都给我们，我们就饶了你。
Provided that the shop assistant knocks 40% off this suit, I shall take it home.
假如导购给我打六折，我将买下这套西服。
Please call us in case you get any difficulty.
万一你有什么难处，就一定给我们打电话。
2. The aircraft was able to fly over the endless white plains without difficulty.
飞机可以毫无困难地飞过这片茫茫无迹的白色原野！
知识点：without difficulty意为“毫无困难地”，做状语。
3. It could only get over the mountains if it rose to 10,000 feet.
只有在飞机飞至10,000英尺的高度时，它才能飞过这些山头。
知识点：本句包含“only + if”的结构，意为“只有……才……”。
The soldiers could only march to the next town if the rain stopped.
这些士兵们只有等到雨停了才能够行军到下一个城镇。
4. The plane was then able to rise and it cleared the mountains by 400 feet.
于是飞机上升了，在离山头400英尺的高度飞越了过去。
知识点：by在此句中表示相差的距离，并非表被动或者“在……旁边”。
