问题补充：
--- Could you tell me? --- Certainly. In 20 minutes.
A. when will the train to Beijing leave
B. when the train to Beijing would leave
C. when the train to Beijing left
D. when the train to Beijing will leave
答案：
A. when will the train to Beijing leave
B. when the train to Beijing would leave
C. when the train to Beijing left
(答案→)D. when the train to Beijing will leave
解析：在复合句中用作宾语的从句叫宾语从句。1．语序无论主句是陈述句还是疑问句，宾语从句都必须使用陈述语序，即“主句＋连词＋宾语从句（主语＋谓语＋……）”句式。2.时态：1·主句用一般现在时，从句可用任意时。2·主句用过去时，从句用过去某个时态。3·主句用过去时，从句是客观真理时，用一般现在时态。3.连接词，分连接副词、连接代词、从属连词。连接词位于主句和从句之间。句意：你能告诉我···？当然可以，在20分钟后。主句是委婉语气，从句根据回答应该用将来时态，故选D。
