13. I don't understand a single word of this letter.

I can't make sense of a single word of this letter.

14. It's pity I am not good at fixing household appliances.

-> I wish I were good at fixing household appliances.
15. “Are you leaving for Hanoi tomorrow?" he said to me.-

> He asked me if I was leaving for Hanoi the next day.
16. He last smoked cigarettes three years ago.

-> He hasn't smoked cigarettes for three years 
17. My parents have just bought a new computer.

-> A new computer has just been bought by my parents.
19.The girl is very friendly. She lives next door to us.

The girl who lives next door to us is very friendly. 
20. The weather was cold, but they decided to go for a picnic.

-> Although the weather was cold,  they decided to go for a picnic.
21. He last visited his home village five weeks ago.

-> He hasn't visited his home village for five weeks.
22. They have done the homework.

-> The homework has been done .
23. I am sorry I don't have a new computer.

-> I wish I had a new computer.
24. They sold their motorbike yesterday.

-> Their motorbike was sold yesterday.
25. "I am writing a letter to Linh." Susan said.

-> Susan said she was writing a letter to Linh.
26. In spite of the heavy traffic, Mr. Smith always went to work on time.

> Although the traffic was heavy, Mr. Smith always went to work on time.

-_-