13. It seems that the further cats fall, the less they are likely to injure themselves.

典型的the more...the more...句型，用定冠词the 修饰形容词或副词的比较级

e.g. The harder you study English, the better you can learn it.

e.g. The more you eat, the fatter you will be.

be likely to do:有可能如何，强调可能性

e.g. In cold winter, We are likely to catch a cold.

special difficulties

总结：常用的双重否定结构（作用：加强肯定语气）

(1) 否定词+fail to（语气较强烈）

比如：never fail to, can't fail to, don't fail to；

(2) 否定的形容词之前加以否定

比如：unreasonable不合理的 -> not unreasonable 不无道理的

e.g. What he said is not unreasonable. 他所说的不无道理。新概念英语青少版

e.g. It is quite common for us to make mistakes.＝It is not uncommon for us to make mistakes. 对我们而言，犯错误是再平常不过的事了。

(3) not + without

e.g. You can't obtain English without working hard. 没有认真学习，你不可能征服英语。

e.g. He doesn't have any meal without meat. 他每餐都要吃肉。

(4) no+not

e.g. There is no cat that doesn't like fish. 没有哪只猫不爱吃鱼。

e.g. There is nothing that he can't do. 没有什么他不能做的。