21. If you do not study, you may fail the test.
22. I will buy you a present if I have enough money.
23. When you run fast, you get tired.
24. If you go to the party, I'm sure you will have a good time.
25. If we buy a car, we will not have to travel by train.
Ответ: 21. do not study; 22. will buy; 23. run; 24. will have; 25. will not have to