1. When does the train arrive? The teacher wanted to know.The teacher wanted to know ________ _____,空格是不是少了一个
问题描述：
1. When does the train arrive? The teacher wanted to know.
The teacher wanted to know ________ _____,空格是不是少了一个
答
如果是将这两个句子融合成一个复合句，我认为是这样变形，主要考察when,(when the train arrive).
答
when the train arrived，这是把两个句子合成一个宾语从句，从句要用陈述语序，即主语+谓语
答
The teacher wanted to know 【when the train arrived】
答
是的，貌似最少要三个吧。
可以是 the teacher wanted to know when it arrived=the teacher wanted to know its arriving time=the teacher wanted to know the time of the train's arrival...etc.
如果是作业，建议你留着问下老师或者跟同学交流下，个人意见，希望帮你解决问题。