ItALSO POSSIBLE: This was themy first time I felt I’d witnessed suchexperience of a cultural disconnect. ALSO POSSIBLE: This was my first experience of a cultural disconnect.

It was the first time I felt I’ had witnessed such a cultural disconnect. It was the first time I felt I had witnessed such a cultural disconnect.

Replaced the contraction “I’d” with “I had” for clarity and formality in writing.

When I wasMY PREFERENCE: One day in elementary school, our soccer club held a penalty-kick showdown between a Brazilian goalkeeper and a Japanese kicker. MY PREFERENCE: One day in elementary school, our soccer club held a penalty-kick showdown between a Brazilian goalkeeper and a Japanese kicker.

NOTE: This is event was only time, the phrase "One day in elementary school" is better than "When I was in elementary school."

In the crowd was another Brazilian classmate, and as a fellow countryman he was enthusiastically cheering on theed for the goal keeper. In the crowd was another Brazilian classmate, and as a fellow countryman he enthusiastically cheered for the goal keeper.

In the crowd was another Brazilian classmate, and who, as a fellow countryman he, was enthusiastically cheering on the keeper. In the crowd was another Brazilian classmate who, as a fellow countryman, was enthusiastically cheering on the keeper.

Added “who” to connect the clauses more smoothly, and commas around “as a fellow countryman” improve readability by marking the parenthetical phrase.

Looking back, I realize now that he’ had unwittingly committed a faux pas. Looking back, I realize now that he had unwittingly committed a faux pas.

Replacing the contraction “he’d” with “he had” makes it clearer and more formal.

As the kicker placed the ball and prepared to strike, another Brazilian classmate shouted out at the kicker, “Miss it!” The Japanese kids watching all frowned and shook their heads, saying things like, “That’s not right,” “You shouldn’t say that,” and “That’s no good.” Caught off guard by the Japanese kids’ reaction, the Brazilian boy seemed bewildered and didn’t yet understand yet what was wrong with his remark, and I could see him growing increasingly sullen. As the kicker placed the ball and prepared to strike, another Brazilian classmate shouted out at the kicker, “Miss it!” The Japanese kids watching all frowned and shook their heads, saying things like, “That’s not right,” “You shouldn’t say that,” and “That’s no good.” Caught off guard by the Japanese kids’ reaction, the Brazilian boy seemed bewildered and didn’t understand yet what was wrong with his remark, and I could see him growing increasingly sullen.

As the kicker placed the ball and prepared to strike, another Brazilian classmate shouted out ato the kicker, “Miss it!” The Japanese kids watching all frowned and shook their heads, saying things like, “That’s not right,” “You shouldn’t say that,” and “That’s no good.” Caught off guard by the Japanese kids’ reaction, the Brazilian boy seemed bewildered and didn’t yet understand what was wrong with his remark, and. I could see him growing increasingly sullen. As the kicker placed the ball and prepared to strike, another Brazilian classmate shouted to the kicker, “Miss it!” The Japanese kids watching all frowned and shook their heads, saying things like, “That’s not right,” “You shouldn’t say that,” and “That’s no good.” Caught off guard by the Japanese kids’ reaction, the Brazilian boy seemed bewildered and didn’t yet understand what was wrong with his remark. I could see him growing increasingly sullen.

“Shouted out at” is a bit awkward; “shouted to” is clearer and more natural when referring to speaking toward someone. The last sentence is a bit long and contains two independent clauses joined by “and.” Splitting into two sentences improves clarity and flow.

After we graduated, I lost touch with both the Brazilian and Japanese kids, so I have no idea what kind of adults they became, but somehow that moment has stayed with me ever since. After we graduated, I lost touch with both the Brazilian and Japanese kids, so I have no idea what kind of adults they became, but somehow that moment has stayed with me ever since.

After we graduated, I lost touch with both the Brazilian and Japanese kids, so I have no idea what kind of adults they became, b. But somehow, that moment has stayed with me ever since. After we graduated, I lost touch with both the Brazilian and Japanese kids, so I have no idea what kind of adults they became. But somehow, that moment has stayed with me ever since.

The original is a run-on sentence with two independent ideas joined by “but.” Breaking it into two sentences improves readability. Added a comma after “somehow” for natural pause.