When do I have to use 做/当 instead of 是? I know 做/当 in some contexts may mean "to be", as in:


  
  他现在做老师了。 He is a teacher now.  
  她愿意做我女朋友。 She accepted to be my girlfriend.  
  我毕业后想做经理。 After I graduate I want to be a manager.
  


I have a hard time explaining to my students when they have to use 做/当 and can't use 是. Any ideas?
In my opinion, 是 expresses a state 一种状态 and it's used to state a fact，while 做/当 denotes an act 一种动作, meaning 充当，担任.  For example, 我是老师 clarified the fact you are a teacher, while 我做/当老师 denotes you want to act as a teacher even if sometimes it's not really your job.