8.I didn't prepare for today's lesson. So it difficult for me to understand the teacher's explanation.
-> It wouldn't be difficult for me to understand the teacher's explanation if I had prepared for today's lesson.

9.I don't want them to be upset, so T've decided not to tell them what happened.
-> They would be upset if I decided to tell them what happened.

10.Martin failed his driving test last week because he was nervous.

 ->If Martin hadn't been nervous, he wouldn't have failed his driving test./But for his being nervous, Martin wouldn't have failed his driving test last week.

11.Go right now or you'll be late for the train.

->If you go right now, you won't be late for the train.

12.Sally doesn’t know what she has to do for homework because she was absent from school on Friday.

If Sally had not been absent from school on Friday, she would know what she has to do for homework.

13.Olga and Ivan weren’t paying attention, so they didn’t see the sign marking their exit from the highway.

If Olga and Ivan had been paying attention, they would have seen the sign marking their exit from the highway.

14.Paulo is working on two jobs right. now, so he doesn’t have time to help you with your remodeling.

If Paulo wasn’t/ weren’t working on two jobs right now, he would have time to help you with your remodeling.