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 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 theater.
If I got a ticket, I would go to the theater.
I I had got a ticket, I would have gone to the theater.