1.During my most recent trip ,I came across a wonderful antique store when (I was) wandering in the old quarter of the city.加个when,后面省略了I was
2.independence 是不可数名词,即便是their,也用independence
例如：African territories eventually obtained their independence.
非洲领土最终获得了独立.
3.by + 时间,是固定用法,表示“到.的时候”
by the sixth century B.C.they were no longer used in battle:到公元前6世纪的时候战车就不用了
新概念里有一句话：By the time you read this,the Hubble will have sent thousnads and thousands of pitcures.到你读到这篇文章的时候,哈伯望远镜已经传送了成千上万的图片.