4，If it is not, and you are likely to get seasick, no form of transport could be worse.

if it is not后面省略了calm

be likely to意为有可能

e.g. He is not likely to come.他不见得会来。

本句中，连词and用来连接一个表示结果的句子，意为：那么，则。And的这一用法尤其在口语中很常见新概念

e.g. If you don't make anything wrong, and you need have no blame.如果你没做错事，那么你不必自责。

Seasick指"晕船"，类似的表示有：airsick/carsick晕机/车

No form of transport could be worse (than it) 为带有否定词的比较级表示最高级。

5，But nothing can match them for speed and comfort.

Match意为与......相匹敌（be equal to）

e.g. No one can match him in singing.

For此处意为"就......而言",相当于as far as.....be concerned.

6, Travelling at a height of 30,000 feet, far above the clouds, and at over 500 miles an hour is an exhilarating experience.

句中at a height of 30,ooo feet和at over 500 miles an hour为两个并列的介词短语作状语，修饰动名词主语travelling，并与其一同构成动名词短语，在句子中作主语。

文中far above the clouds作插入语，对at a height of 30,000 feet作补充说明。