Lan is the most intelligent person in my class.

This exercise was so difficult that we couldn't do it.

Had it not been for your help, I couldn't have passed the exam.

"When will you come back, Nga?" → I asked Nga when she would come back.

No one has bought this house. → This house hasn't been bought by anyone.

She didn't take an umbrella so she got wet. → She wouldn't have gotten wet if she had taken an umbrella.

It was a mistake for him to buy that house. → He shouldn't have bought that house.

Although Nam was very tired, he tried to finish his work. → However tired Nam was, he tried to finish his work.

We couldn't go camping because of the storm. → The storm prevented us from going camping.

It's ages since I last talked to him. → I have not talked to him in ages.

The book was so interesting that we couldn't put it down. → It was such an interesting book that we couldn't put it down.

He said to me "Don't stay up late." → He told me not to stay up late.