When I think of it now, I am amazed at how much courage it must have taken ...(Para. 5)
Meaning: When I think of it now, I feel greatly surprised at how much courage it must have taken ...
Notice that "be amazed" is used together with "at" or "by". Look at the following example:
I was amazed at / by her calmness. 她的鎮靜自若令我吃驚。
But "be amazed" can also be followed by to-infinitive or that-clause. In these two cases we can not use any preposition:
We were amazed to hear that she passed the test without much difficulty. 聽說她輕而易舉地通過了考試, 我們都很驚奇。
I was amazed that you were leaving. 你要離開, 我很吃驚。
If "be amazed" is complemented by a how-clause, use of the preposition "at" or "by" is a choice:
You would be amazed (at / by) how difficult the job was. 你要是知道那件任務有多難, 你會大吃一驚的。
In the above sentence, the preposition "at" or "by" can be left out in written English.