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?