He went to the United States with one of his best friends from when he was a child.

【翻译】从他的孩提时代起，他就和一个最好的朋友一起去了美国。

【分析】从语法分析，when he was a child 是一个名词性从句，作介词from 的宾语（即：宾语从句）。from when he was a child 在意思上等于 from his childhood。

还可以理解为：from the time when he was a child，这时的when he was a child，成为time的定语从句。

▲from有一个比较特殊的用法，就是后跟when，where等引导的从句。

From when he was a young man, he had suffered from a chronic lung illness. 从他年轻时起，他受到慢性肺部疾病的折磨。

I remember everything from when we first went to sea together. 打我们头一趟一同到海里去的时候起，什么事我都记得一清二楚的。

The shops were at a distance from where they lived. 商店离他们住所有一段路。

I could see him from where I was hiding. 我从我藏身的地方可以看到他。

▲关于句子有无from的区别：

1. He went to the United States with one of his best friends when he was a child.

2. He went to the United States with one of his best friends from when he was a child.

以上两句都对。但是，意思有差别：

句1，“去美国是在很小的时候”，说明在过去某个时间点所发生的事情；句2，“从孩提时代起就去了美国”，说明某人从某个时间开始，就做某事了，强调了某事发生的时间很早，含有since的意思。例如这两句：

From when he was a young man, he had suffered from a chronic lung illness. 从他年轻时起，他受到慢性肺部疾病的折磨。

如果没有from，就不知道他后来一直遭受疾病的折磨，只知道年轻时的情况。

I remember everything from when we first went to sea together. 打我们头一趟一同到海里去的时候起，什么事我都记得一清二楚的。

此句的from when we first went to sea together作定语，修饰everything。如果没有from，when we first went to sea together成为修饰谓语remember了，语法不通，意思也讲不通。