几个例子:when doing his homework,he heard someone singing当正在做作业的时候,他听到有人唱歌.When (you are) finished with your work,call me当做完了作业,给我打电话.I don't know when to go 不知道什么时候...
when doing--something else's happening/ concurrently. e.g. When doing this question, you have to look at the previous answer too! when done--to be finished off / if...then kind of situation