why we（主语） cannot do（谓语） that（宾语）.
从句中有主语和宾语,那么不缺主语和宾语,缺少状语,再加上先行词是reasons,表原因,所以用why.
where flowers are seen all the year round.
如果是被动的不会划分,那先还原为主动的来看看.
flowers are seen all the year round.（这里省略了主语,原因可查被动语态省略主语的原因）
那么还原成主动语态就是：
We see flowers all the year round.
所以where flowers are seen all the year round 从句里不缺少主语和宾语,先行词是a city,所以要用where .
如果这样解释你不能理解,那么你记住从句后如果有主语,就是在动词或be动词前面有名词或代词,那么从句就不缺少主语和宾语,就缺少状语,看看先行词是什么就可以了.
when he taken away from his mother and sold .
这句你肯定抄错了.从句是被动语态.所以应该是when he was taken away from his mother and sold .
先行词是the day ,表示时间,所以用when,原因同上面的被动语态一样.