这句话非常地道，重点是 take one look at someone 和 before I did。

She took one look at me and knew what was happening before I did.

自然翻译：

她看了我一眼，就在我自己意识到之前，已经知道发生什么事了。

也可以更口语地翻译：

她一看我就知道出事了，而且比我自己还早意识到。

① take one look at me ⭐⭐⭐⭐⭐

这里不是单纯的“看了一眼”。

take one look at someone

= 一看某人，就马上发现/判断出什么

例如：

I took one look at him and knew he was lying.

我一看他，就知道他在撒谎。

所以：

She took one look at me

= 她一看我

后面的 and knew... 就是“一看就知道”。

② what was happening

what was happening

= 正在发生什么事

注意这是一个宾语从句：

She knew [what was happening].

= 她知道 正在发生什么事。

③ before I did ⭐⭐⭐⭐⭐

这里特别容易让中国人困惑。

完整形式其实是：

before I knew what was happening

但英语为了避免重复，把后面的内容省略了：

before I did

这里 did 代替：

knew what was happening

所以：

She knew what was happening before I did.

= 她知道发生什么事，比我自己知道得还早。

不是：

❌ 在我做某事之前。

这里的 did 可以理解成：

“我也知道了这件事”

整句话的画面

前面的情景是妈妈血糖很低。

She took one look at me

她一看我的状态，

and knew what was happening

马上就知道发生了什么，

before I did.

甚至比我自己还早意识到。

所以这句话非常适合记住一个结构：

A knew it before B did.

= A 比 B 更早知道/意识到这件事。

例如：

She knew I was sick before I did.

= 她比我自己还早意识到我生病了。

这里的 did 都不是具体的“做”，而是在代替前面已经出现过的动词。