once a week是頻率副詞,表示的是一種不斷重復(fù)的行為頻度,往往放在句子中間，實義動詞之前，非實義動詞之后，在否定句中，有的頻度副詞位于否定詞之前或之后均可，而有的頻度副詞則必須位于否定詞之后，還有的頻度副詞卻必須要位于否定詞之前。

頻率副詞等級

all the time 100%；

almost，always 90%；

generally，usually 80%；

often，frequently 60-70%；

as often as not 40-50%；

sometimes，at times 20-30%；

occasionally，from time to time 10%；

hardly，seldom，rarely 5%；

never 0%。

3、頻率副詞用在be動詞后

He was always late.他以前經(jīng)常遲到。

4、頻率副詞用在實意動詞前

My father usually walks home.我的父親經(jīng)常步行回家。

1、How many skateboard classes once a week?

2、Occurring, appearing, or done once a week or every week.

3、Go once a week if you can.

4、Lung cells were replaced once a week.

5、She does the aerobics once a week.

6、What you should do: Run Yasso 800s once a week.

7、We used go to the movies about once a week.

8、Hence clean the ears once a week with cotton buds.

9、They also check for diseases or pests once a week.

10、Proofread the English Ad once a week by Email.

來源：高三網(wǎng)