頻率。

every other + 單數名詞 = every two 複數名詞。

every other day/year. = every two days/years 每隔一天/年，兩天/年一次

every three days 每隔三天。

once a + 單數時間詞 (once a week/month/year etc. － 一星期/一個月/一年等一次) 及 

once every + 複數時間詞 (once every two days/three years etc. － 每兩天/三年等一次) 

來表示事情發生的頻率。例如：

• We go for dinner together once a week. (我們一個禮拜會共進晚餐一次) (正)
• Tom visits his parents only once every two or three months. (湯姆每兩三個月才探望他父母親一次)
• It seems like every other shop here is a café.
  這裡似乎每隔一間店就是一家咖啡廳。

once a day
twice a day
three times a day