Who are you writing to的中文意思?用Whom are you writing to 可以吗

中文意思是：你给谁写信呢?
关于who 为主格,whom为宾格.只能用whom的情况,如：(1) 直接跟在介词后面作宾语时,只能用 whom,而且不能省略.如：She brought with her three friends,none of whom I had ever met before.她带了brought with her three friends,none of whom I had ever met before.3个朋友来,我以前都没见过.(2) 引导非限制性定语从句且作宾语时,who 和 whom均可用,但以用 whom 为佳,此时也不能省略.如：This is Jack,who [whom] you haven’t met before.这是杰克,你以前没见过.