1. We think young people should look smart and so we would like to wear our own clothes. Our teachers believe that if we did that, we would concentrate(集中注意力) more on our clothes than our studies.（虚拟语气，只可以用 if，when 和 after 引导时间从句，不用虚拟语气）

2. He wrote his wife's name on the back of the photos and handed them to a clerk behind the desk in the booth. "When/If you see a small, dark lady..., would you please give her this?" （这个从句主要表达“条件”，when 也可用来表示条件，但 after 不可以表示条件，如果用 after 表示时间，不符合语境。）

3. Many teachers believe that the responsibilities for learning lie with the student. After a long reading assignment is given, instructors expect students to be familiar with information in the reading even if they do not discuss it in class or take an examination.（主从句强调先后关系，而且先后关系明确）