If a group of people pair off, they divide into pairs in order to do something, and if you pair them off, you divide them into pairs.
如果一群人pair off，那么就是说他们是分成两人为一组来做某事的，如果你把他们pair off，那么就是说你把他们分成两两一组。
两人一组结成对;把(一组人)分成几对
The students paired off to practise their conversational skills.
学生们两人一组进行会话技巧练习。
After dinner the boys and girls paired off and went to the dance.
会餐后， 男女青年们成对成对地去跳舞。
