77. There was fog, so we postponed the flight.
=>Because of the foggy weather ,  we postponed the flight.
78. It will be good to see them again.
->I'm looking forward to seeing  them again.
79. I'd prefer not to go out tonight.
=> I'd rather not go out tonight.
81. Why did you allow them to leave in the storm?
-> You shouldn't let them leave in the storm
82. Lan hasn't met her old friends since they left school.
The last time Lan met her old friends was when they left school
83. Although Tom felt unwell, he went on working.
->Despie felling unwell, he went on working.
84. The weather was so lovely that we decided to spend the day on the beach.
It was such  lovely weather that we decided to spend the day on the beach
85. Whose does this bag belong to?
=>Whose bag is this?
87. If he improves his spelling, he will get good grades.
=>Unless  he improves his spelling , he won't get good grades.
88, Tuan failed the exam because he was lazy.
=>Because of being lazy , Tuan failed the exam
89. Learning English isn't very difficult.
=>It's not very difficult to learn English