用括号中动词的正确形式填空,说明为什么这样填
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)