he could hum songs when he was eight years old.这句话为什么要用could

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he could hum songs when he was eight years old.这句话为什么要用could,而不是can那不就可以理解为
他以前会哼歌,

could某些时候可以当成can的过去式
不是以前会现在不会只是在强调在他八岁的那个时间点开始会唱了
就好比说 he could ski when he was three 一样只是在强调那个过去的时间点
再问：过去式不是发生在过去，结束在过去吗?
再答：是，但是还有个前提是有过去具体的时间点比如 three days ago last week i went to paris last summer i have been to paris 如果时间不明确发生在过去就用现在完成时了

he could hum songs when he was eight years old.这句话为什么要用could he could hum songs when he was 4 years old 句型转换 He could hum songs when he was five. When he was a small boy,he could hum songs and difficuld pie Li Yundi could hum songs and___when he was very young. He could speak English when he was four years old. When he was five years old ,he could swim ( )( )( )( )( )fiv he could hum songs and difficult pieces of music!翻译这句话! Jim could sing when he was five years old.（改为同义句） Jim could he could hum songs and difficult pieces of music he could hum songs and diffcult pieces of music . 同义句转换;He could speak English when he was four years old He _