频度副词：
一般放在be动词之后或者助动词与主要动词之间,但sometimes、often等还可以放在句首或句尾,usually可放在句首,once可放在句尾,twice、three times等一般放在句尾。
如：Sometimes I get up early.(我有时起得早)
The workers usually have lunch at the factory.(工人们通常在厂里吃午饭)
Take this medicine twice a day.(这种药一天吃两次)
学识谷
