19. Unless somebody waters them, these flowers will die. 

24. If it had been fine yesterday, we would have had a picnic. 

25. Unless we leave now, we will miss the train. 

26, If the house weren't too expensive, i could buy it. 

27. If i had seen the signal, i would have stopped. 

28. If you pay attention to what the teacher said, you will understand the lesson. 

Unless you pay attention to what the teacher said, you won't understand the lesson. 

29. If he had prepared for the interview, he would have got the job. 

30. If i had known how difficult my job was, i wouldn't have taken it. 

Bn học tốt!!