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のを見て驚いた

was surprised to see

→ の
phrase Rare #34,933
was surprised to see Follows a clause describing what someone saw and expresses surprise at seeing it. The の nominalizes the preceding clause, while を見て驚いた means “was surprised upon seeing.”
JA: 私は彼が一人で料理しているのを見て驚いた。
EN: I was surprised to see him cooking by himself.

私は中国人が石炭を燃やしているのを見て驚いた。

I was surprised to see Chinese people burning coal.

全生徒がプロムの夜礼儀正しく振る舞っているのを見て驚いた。

I was surprised to see all the students behaving themselves on prom night.

コンサートにあんなに多くの人がいるのを見て驚いた。

I was surprised to see so many people at the concert.

彼ら全員が、私がそこにいるのを見て驚いた。

They were all surprised to see me there.

彼らはみんな、私がそこにいるのを見て驚いた。

They were all surprised to see me there.