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！
I was surprised to find out Phoenix has the 6th biggest population of all the cities in America.
この場合、次のような言い方ができますよ。
ーI was surprised to find out Phoenix has the 6th biggest population of all the cities in America.
「フェニックスがアメリカのすべての都市の中で6番目に人口が多いことを知って驚いた」
Phoenix has the 6th biggest population. で「フェニックスは、6番目に人口が多い」と言えます。
ご参考まで！
アメリカ合衆国 
カナダ 
