第一部分
3.they wrote letters of thanks to all their friends
这句错了,应该是改成：letters of thanks were written to all their friends.
其他都正确.
第二部分 (被动变主动) 这部分要注意时态.
1.hundreds of workers are building many factories now
2.her brother will meet her at the station
3.（I） have found the watch
4.a lot of friends from other countries visit the university
5.（people） made it in 1938
第3题,第5题,由于主语被省略了,所以只能靠自己加上一个主语,只要句子解释得通就可以了,我已经用括号注明了.