3.Precautions of this sort are necessary,for it is impossible to foretell the exact nature of difficulties which will confront the pot-holer.

结构分析：for 连接两个并列句，第二个并列分句中which引导的定语从句修饰difficulties

语汇注释：新概念英语第二册视频

precaution  n.预防措施  Eg: We have to save our precautions of insuring my car.

take steps/actions 采取措施

4.The cave might never been discovered had not the entrance been spotted by the distinguished French pot-holers,Berger.

背诵句型

结构分析：had not the entrance been spotted by the distinguished French pot-holers,Berger是非真实条件句，省略了if。即：如果条件从句中有were ,had 或should ,可以把if省略，把这几个词放到主语之前，构成主谓倒装.

eg:Had he taken a little more time to think,he might have acted more sensibly.

5.They had to edge their way along this,cometimes wading across shallow streams ,or swimming across deep pools.

背诵句型

结构分析：简单句，cometimes wading across shallow streams ,or swimming across deep pools做主语补足语，补充说明该洞穴内的险峻。

词汇注释：edge one's  way 侧身挤过，侧身徐徐向前