Tom is his best friend ,【Tom 是主语,用 who 提问,在变宾语从句中,语序不变.who is your best friend 】
His best friend is Tom 【Tom 是表语,用whom / who 提问,在变宾语从句时,要用陈述句语序who / whom his best friend is 】
Tom is his best friend ,【Tom 是主语,用 who 提问,在变宾语从句中,语序不变.who is your best friend 】
His best friend is Tom 【Tom 是表语,用whom / who 提问,在变宾语从句时,要用陈述句语序who / whom his best friend is 】