答案：
小题1:My dad has gone to the USA on business.
小题2:I have kept this book for two and a half weeks.
小题3:They get/are used to keeping in touch with each other by email
小题4:Spring is the best time to visit Chinese gardens.
小题5:By the way, how long does it take you to fly to Hong Kong?
题目分析：
小题1:分析句意：出差on business; 去：不在说话地用have gone to , 故填：My dad has gone to the USA on business.
小题2:分析句意：两个半星期：two and a half weeks; 出现段时间表示借时要转换为延续性动词keep,故填：I have kept this book for two and a half weeks.
小题3:分析句意：习惯于做某事：be used to doing sth. 保持联系：keep in touch with,故填：They get/are used to keeping in touch with each other by email.
小题4:分析句意：最好的时候：the best time,故填：Spring is the best time to visit Chinese gardens.
小题5:分析句意：.顺便问一下 :by the way,故填：By the way, how long does it take you to fly to Hong Kong?