第一段第二句我觉得可以写成The student union of our school will organize an activity in recent days and you can join us as volunteers.第二段第一句This activity will not only let you do something meaningful but also extend your compentency.有一些语法错误，比如 will , let 后面是要跟原形的，不加to多用一些短语，比如If you want to join us=If you have anintention to join ...
