1.当表示时间的名词前有(形容词等限定词)时,不用介词
2.once a week, every day, three times a month等
3.频度副词在句中习惯上位于be动词、助动词等之后,行为动词之前
4.sometimes,often
Sometimes she writes to me. =She writes to me sometimes. 她有时候给我写信。
She writes to me often. 她经常给我写信。
好像是限定词吧
频度副词在句中习惯上位于be动词、助动词等之后,行为动词之前。
once a week,
often