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. 她的镇静自若令我吃惊。
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. 听说她轻而易举地通过了考试, 我们都很惊奇。
I was amazed that you were leaving. 你要离开, 我很吃惊。
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. 你要是知道那件任务有多难, 你会大吃一惊的。
In the above sentence, the preposition "at" or "by" can be left out in written English.