必做题目：
1.Sometimes I wish I am living in a different time and a different place.
2.Had electronic computers not been invented,many problems on space flight could not be solved.
选做题目：
It is obligatory that every young man serves in the armed forces.
必做答案：
1.应该将am living改为were living.该题考察wish+that从句的虚拟表达法。Sometimes是一个频率副词，表示现实里经常性的动作，显然这里需要的动词虚拟式为过去时形式。
2.应该将could not be改为could not have been。该题考察非真实条件句虚拟语气的用法。根据从句可知此处是与过去事实相反的虚拟，且solve与many problems之间为被动关系，因而要用could not have been solved的表达形式。
选做答案：应该将serves改为serve。该题考察obligatory之类的形容词引导主语从句，从句中要使用虚拟语气，从句中动词形式为should+动词原形，should可以省略。