• I haven't spoken to Jack since I sold him my old car.

• It took 3 days for the train to Ha Noi.

• It's time he went home.

• If Mary had had spare time, she would have visited her grandmother.

• These books were said to have been published in 1998.

• I would rather you didn't smoke in here.

• Hardly had I put the phone down when the boss rang back.

• I wish you had passed your exam.

• The river is not warm enough for her to swim in.

• The students suggested buying some English books.

Here are the completed sentences, along with explanations for each:

1. I last spoke to Jack when I sold him my old car.

   • I haven't spoken to Jack since I sold him my old car. (This sentence uses the present perfect tense to express an action that started in the past and continues up to the present.)

2. The train to Ha Noi lasted 3 days.

   • It took 3 days to go to Ha Noi by train. (This sentence focuses on the duration of the journey.)

3. He should go home now.

   • It's time for him to go home now. (This sentence expresses the idea that it is appropriate for him to leave now.)

4. Mary didn't visit her grandmother because she didn't have spare time.

   • If Mary had had spare time, she would have visited her grandmother. (This is a second conditional sentence, expressing a hypothetical situation in the past.)

5. People said these books were published in 1998.

   • These books are said to have been published in 1998. (This is the passive voice, expressing the same idea in a different way.)

6. I would prefer you not to smoke in here.

   • I would rather you didn't smoke in here. (Both sentences express a preference.)

7. I had only just put the phone down when the boss rang back.

   • Hardly had I put the phone down when the boss rang back. (This is a more formal way of expressing the same idea, using inversion.)

8. What a pity you failed your exam.

   • I wish you hadn't failed your exam. (This expresses a regret about a past event.)

9. She won't swim in the river because it is too cold.

   • The river is not warm enough for her to swim in. (This sentence focuses on the water temperature.)

10. The students suggested buying some English books.

• The students suggested that they should buy some English books. (This is a more complete way of expressing the suggestion.)