用括号中动词的正确形式填空，说明为什么这样填

Question

1.When I lost my wallet,I never expected it to ____(return)
2.I didn't receive the letter.I think it _____in the post.(must lose)
3.The PLA____on August 1st,1927(found)
4.He left home as a young man and ______since (not see)
5.The thieves ____50,000 from the shopping mall.(steal)
6.Japanese ___as the second language in China (learn)

Answer 1

be returned 表示被动。被还回来
must have been lost表示对过去的推测，上文是过去时，所以表示过去一定是丢在，，，
was founded表示被动。被成立
hasn't been seen因为后面的介词since 意思是从那时到现在，要用现在完成时。
have stolen句子中没有时间状语，所以是表示现在的结果。要用现在完成时。
is learned表示一般情况，这种语言一直在中国被学。所以用一般现在时。

Answer 2

1.When I lost my wallet,I never expected it to【be returned】.
解释：被送回来---被动语态
2.I didn't receive the letter.I think it【must have been lost】in the post.
解释：同上，信“被丢失了”
3.The PLA【was found】on August 1st,1927.
解释：同上，被发现了
4.He left home as a young man and【was not seen】since.
解释：没有再“被见到”
5.The thieves【stole】50,000 from the shopping mall.
解释：偷的过去式，have/had stolen也可以。
6.Japanese【is learnt】as the second language in China.
解释：日语在中国”被学“，被动