~できた　＝ could か

could は、can の過去（～できた）という意味で使えるが、特定の時点を示す語句と共に使う時には be able to を使う方が好まれる。

There's a nuance when you talk about a specific occasion in the past. If you want to talk about a one-time event or a particular incident, it's more common to use "was/were able to" or "managed to".

For instance, "I was able to catch the bus yesterday just in time" would generally be preferred over "I could catch the bus yesterday just in time". The second sentence might sound strange to native English speakers because "could" is typically used for general ability, not specific instances.

So, "could" can be used with specific times in the past, but for specific occasions or events, "was able to" is often a better choice.

an answer generated by Chat GPT 4, Bold by the writer

My Question:
Then " I was able to win the first place in the 100m race when I was the 1st grader." is preferable to "I could win the first place in the 100m race when I was a 1st grader." Right?

Chat GPT4 's answer:
Yes, that is correct. "I was able to win the first place in the 100m race when I was a 1st grader" is indeed more appropriate than "I could win the first place in the 100m race when I was a 1st grader."
"I was able to" is better suited for expressing a specific achievement, while "could" is typically used for general abilities or possibilities rather than a specific instance. Therefore, the former sentence is more suitable when discussing a specific event or accomplishment, such as winning a race.
So, the preferred sentence is: "I was able to win first place in the 100m race when I was a first grader."

In Japanese, this could be translated as: "私が一年生のとき、100m走で一位を取ることができました。"