频度副词:频度副词包括确定性频度副词和非确定性频度副词
一．确定性频度副词：once，twice，three times a day weekly，daily，fortnightly，monthly，yearly，annually
1．确定性频度副词通常放在句尾
I saw my girlfriend once a week
he visited his cousins fortnightly. 他每两周拜访他的兄妹们一次。
They celebrate their wedding anniversary annually. 他们每年庆祝一番结婚纪念日。
2．但在复合句中，由于句中不只存在一个动词，因此应根据频度副词所修饰的对象确定其相应位置，以免产生歧义
Once a monthly,we visit our daughter who’s at Harvard University.
我们每个月去看望在哈佛大学上学的女儿（once a month 用在句首，因此根据其就近原则，应理解为它所修饰的对象是visit，而不是is）
We visit our daughter who’s at Harvard University once a month.
我们去看望每个月去一次哈佛大学的女儿（once a month 用在句尾，这时应该理解为它所修饰的对象是is，而不是visit）
二 ．非确定性频度副词：always, generally, usually, often, sometimes, seldom, never, ever
1．在肯定句和疑问句中的非确定性频度副词通常放位于be动词之后，谓语动词之前或复合谓语之间。
I usually get up at 6: 00 in the morning.我通常早上六点起床。
He always goes to school by bike.他总是骑车去上学
We hardly ever see Mr Lee these days
I was never very good at maths。
I don’t usually get up before 9:00 on Sundays.
2．非确定性频度副词也可放在句首表示特别强调或对比
Sometimes we get a lot of rain in August. 有时在8月份雨水很大。
Quite often the phone rings when I’m in the bath. 电话经常在我洗澡时响
Usually I get up early. 我平时起得早。
Never did he think the book would be finished so soon. 他可没想到这本书会这么快看完。