I was surprised to learn that Phoenix is the 6th most populous city.
I was surprised when I learned that Phoenix is the 6th most populated city.
質問ありがとうございます。
こう言えますよ、
❶I was surprised to learn that Phoenix is the 6th most populous city.
(フェネックスは人口が六番目に多い都市と知って驚いた)。
Many are surprised to learn that Phoenix is the 6th most populous city.
(フェネックスは人口が六番目に多い都市と知って驚く人が多い)。
❷I was surprised when I learned that Phoenix is the 6th most populated city.
(フェネックスは人口が六番目に多い都市と知ったとき驚いた)。
*Most populous/ most populated は「最も人口が多い」です。
参考になれば嬉しいです！
アメリカ合衆国 
