1. 请问He swam on for what felt like at least twenty mi（= meters）.

【答】这个句子，可以理解为此句的变式：He swam on for at least 20 meters.

（1）on：表示“继续”之意；swam on：意为“继续前游”。

（2）for + 一段距离：表示游泳的距离。这跟for + 时间段，表示动作“持续的时间”，是一个道理。

（3）at least 20 meters：是个名词词组，我们可以用what引导的名词性从句（what felt like at least twenty meters）代替它，因为名词性从句的本质就是起名词的作用。只是意思有所变化，区别是：

for at least 20 meters：游泳至少20米。（实际在20米之上）

for what felt like at least 20 meters：游泳感觉起来至少20米。（实际多少不清楚）

其实，这个用法跟你下句的with what seemed to be a jet of boiling water，完全是一个类型。

我再举个类似的例句：He stayed there for what was/felt like one year. In fact, he stayed for only a day.

介词后接what引导的名词性从句，英语中比比皆是：

In no way am I responsible for what has happened. 我决不对发生的事情负责。

He gave a description of what he had seen. 他描述了他所见到的一切。

She didn't attend to what I was saying. 她对我所说的话未加注意。

2. 还有请问各位老师,下面这句话中for where it struck them 的for是什么成分？起什么作用？

A large bubble issued from his mouth, and his wand, instead of sending sparks at the grindylows, pelted them with what seemed to be a jet of boiling water, for where it struck them, angry red patches appeared on their green skin.

【答】这个句子中的for where it struck them，跟上面的问题不是一回事。在这里，for是并列连词表原因，意为“因为”。where it struck them是地点状语从句，修饰angry red patches appeared on their green skin。为了理解，我把语序调换一下：

for angry red patches appeared on their green skin where it struck them.

意思是：“因为，在它击中它们的地方，刺眼的红色斑块出现在它们绿色的皮肤上。”