By the time he was twelve,Edison had began to make a living by himself.
问题描述:
By the time he was twelve,Edison had began to make a living by himself.
这个句子中的by,是直到还是到的意思,还有had能不能换成have?
答
都不是 是在.之前的意思 这句话是过去完成时 所以不能换
在12岁之前爱迪生不得不自己谋生