"When I say that I used to live in Japan, people are always surprised."
The form of おどろく used in the translation is おどろかれる, but I thought that おどろく was already pretty much a passive verb (to be surprised) and it takes が, so I can't quite understand why it would need to be put into the passive form.
1) にほんに すんでいたことを はなすと、いつも おどろかれるんですよ。 = When I say that I used to live in Japan, people are always surprised.
The above sentence is not easy to directly translate. Therefore, instead of getting every word perfect, we made the English sound natural (natural translation/page 18).
There are a couple of words that are omitted in the sentence. Please see the words in parentheses below. It may be easier to understand why the passive verb form is used in the sentence when you see those words and the direct translation below.
2) にほんに すんでいたことを はなすと、(わたしは)いつも(みんなに)おどろかれるんですよ。 (Direct translation) = When I say that I used to live in Japan, I always get surprised by people.
The direct translation doesn't sound natural.
In the Workbook Activities section, there is a "Bonus Practice" for the passive verb form. (page 221) Please practice making sentences there and ask us again if you have further questions. がんばってください :)