Who was sitting behind him 谁坐在他的后面.我要回答的是:一个年轻人和一个年轻女人坐在他的后面
1个回答

有两个小错误,一是这句话首次提到年轻人和年轻女人,不应该用定冠词 the,要用不定冠词.二是助动词要用复数,因为主语是 a young man and a young woman, 加起来是两个人,所以要说 were sitting. 复合主语用复数的例子还有 One egg and one lemon are required to make the cake. 注意,并不是所有“xx and xx”的主语都是复数,如果主语指的是一个人或一件事,动词还是要用单数,比如 Strawberries and cream is my favorite dessert, 其中strawberries and cream 指一种甜食,所以当单数用.

正确句型是 A young man and a young woman were sitting behind him.

你的时态是正确的,和问句一致就行.