. The dress is so expensive. She can not buy it. 

=> If……………the dress wasn't so expensive, she could buy it…………………………… …………………..………..

11. She doesn’t have an umbrella, so she gets wet.

-->If………she had an umbrella, she wouldn't get wet……………………………………………………….…
12. You are so nervous because you drink too much coffee.

 --> If……………you didn't drink too much coffee, you wouldn't be so nervous………………………………………………..…… 

13. I’ll call the teacher if you don’t leave alone.

 -->Unless …………you leave alone, I'll call the teacher…………………………………………..…………

14. I don’t tell you because you don’t ask me.

-->If  ………………you asked me, I would tell you………………………………………………………

15. If you don’t ask me for the car, I will not lend it to you.

-->Unless …………you ask me for the car, I won't lend it to you………………………………………………………