he said :" I could swim when i was very yong" 若对怎么变间接引语

这句话对,除了打字错误yong---->young
he said he could swim when he was very young
再问： 我想问一下 为什么这里的 was 不往后退一个时态 直接变间接不应该变时态吗
再答： 不必，
再问： 为什么
再答： 若原句为一般现在时：he says :" I could swim when i was very yong"，改成间接引语，还是 he says he could swim when he was very young，对吗？
再问： 还是有点不明白