the thieves dropped the torch and ran away as quickly as they could.不理解后半句的结构。

状语(adverbial)是句子的重要修饰成分。状语是谓语里的另一个附加成分,,从情况,时间,处所,方式,条件,对象,肯定,否定,范围和程度等方面对谓语中心进行修饰或限制。或者说状语是修饰动词、形容词、副词等的句子成分。
这里的as quickly as they could是一个固定结构，表示尽可能的快
修饰 ran
could在这里是can的过去式而已

as……as one can =as …… as possible 表示尽可能……
I would like to see him as soon as I could.
我想尽可能早一点见他。

Tell me the good news as soon as possible.
尽可能早得告诉我好消息。

You should come here as early as possible.
你应该早一点回来。

We have to run as fast as we can.
我们得尽可能快的跑。

could这里只是单纯的can 的过去时哈，因为前面是dropped,所以奠定了整句话的时态——一般过去时。

as+形容词/副词+as+sb+can/could 是一个固定结构，在本句中could 是过去式。