YOU SAID:

As I supposed Americans aren't good at Mario Kart

INTO JAPANESE

アメリカ人はマリオカートが苦手だと思っていたので

BACK INTO ENGLISH

Americans thought they weren't good at Mario Kart

INTO JAPANESE

アメリカ人はマリオカートが苦手だと思っていました

BACK INTO ENGLISH

Americans thought they weren't good at Mario Kart