1 if we receive a telegram from him, we won't worry. if we received a telegram from him, we wouldn't worry. if we had received a telegram from him, we wouldn't have worried.
2 if you don't work systematically, you will fail the exam. if you didn't work systematically, you would fail the exam. if you hadn't worked systematically, you would have failed the exam.
3 if i live in London, I will visit big Ben every month. if i lived in London, I would visit big Ben every month. if i had lived in London, I would have visited big Ben every month.
4 if i get a ticket, I will go to the theatre. if i got a ticket, I would go to the theatre. if i had got a ticket, I would have gone to the theatre.