我只知道他当时在西方的一个国家读书,可不知道是哪个国家weonlyknew（that）hewasstudyinginawesterncountry,butIdidnotknowwhatcountryhewasin1：后面为什么要使用did?过去式不是在前面从句中已

Question

我只知道他当时在西方的一个国家读书,可不知道是哪个国家
we only knew（ that） he was studying in a western country ,but I did not know what country he was in
1：后面为什么要使用did?过去式不是在前面从句中已经体现了吗?
2:句子中的that是否需要
3：后面的what country he was in 用法 为什么是he was in不是he was
句子比较复杂 希望大家耐心点

Answer 1

1 was == was studying --------was是助动词,在有了上下文的时候采用的“省略法”.
2 did -----强调当时的“知道”,与现在的“感知”无关
3 that----作为宾语从句的连接词完全可以省略
4 he was in Country A ------Country A 被提问-------what country was he in ?
宾语从句： I did not know what country he was in.------就是这样来的,in不能省略.