I saw her play the piano when I was eight.
从汉语句子意思看,是“看过她弹琴”,强调经历,而不是情景,故不用ing形式.
附：英语词典原句：
我八岁时看她表演过.
I saw her play when I was eight. I saw her play the piano when I was eight.
从汉语句子意思看,是“看过她弹琴”,强调经历,而不是情景,故不用ing形式.
附：英语词典原句：
我八岁时看她表演过.
I saw her play when I was eight.