always是频度最高的词，表示动作的重复，状态的继续，中间没有间断。
often意为“经常、常常”，不如usually那么频繁，表示动作重复，中间有间断。
Work always piles up at the end of the year.
年底总是积压一大堆工作。
There's always a lot of traffic at this time of day.
每天这个时候总是有很多来往车辆。
This kind of question often appears in the exam.
这类问题在考试中经常出现。
As often as not, he's late for work.
他上班往往迟到。
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