幾點鐘 (或幾時幾分幾秒)、一天當中的某個時間點、一週當中的某個時間點、特別的節日等時間詞的前面須使用 at。例如：
- I’ll pick you up at the airport at six o’clock. (我六點鐘會到機場接你)
- I woke up from a dream at 4:30 a.m. (我凌晨四點半從夢中醒過來)
- He was released/discharged from (the) hospital at midday. (他中午出院)
- What are you doing at the weekend? (你週末要幹什麼？) － 美式英語通常使用 on the weekend。
- John works at (the) weekends. (約翰每個週末都工作) －美式英語通常使用 on (the) weekends。
- Mary will meet her pen pal at Christmas. (瑪麗將在聖誕節和她的筆友見面)
- At the Lunar New Year, the Lion Dance is one of the most popular traditional performances. (在農曆新年，舞獅是最受歡迎的傳統表演之一)
在非正式情況中，” What time …?” 這個問句的前面通常不用 at。例如：
- What time do you collect the kids from school? (你什麼時候去接孩子放學呢？)
- At what time do you collect the kids from school? － 這句沒有上句來得自然，通常用在非常正式的場合。
日期、星期幾、特別的日子等時間詞的前面須使用 on。例如：
- I moved to Kaohsiung on October 20, 2000. (我在 2000 年 10 月 20 日搬到高雄)
- Kevin had a visit from his mother-in-law on Sunday. (凱文的岳母星期天到他家作客)
- John works (on) Saturdays. (約翰每週六都工作) － Mondays、Tuesdays … 意為「每週一」、「每週二」…，其前的 on 可以且通常省略。
- They’ll give a party for me on my birthday. (他們將在我生日那天為我舉辦派對)
- They were married in two different cities on the same day last year. (他們去年同一天在兩個不同的城市舉行婚禮)
上午、下午、月份、年份、季節、世紀等時間詞的前面須使用 in。例如：
- What time do you usually wake up in the morning? (你早上通常幾點醒來？)
- He’ll do his homework in the evening. (他將在晚上做作業)
- The company was established in October 2010. (這家公司於 2010 年 10 月創立)
- The museum was built in 1643. (這座博物館建於 1643 年)
- It was a cold, sunny day in early spring. (那是早春裡寒冷而陽光燦爛的一天)
- We live in the twenty-first century. (我們生活在二十一世紀)
At 還是 on 呢？
根據上述，at 是用在特別的節日和週末等時間詞之前，但時間詞若是指某個特定的日子或特定的週末，那麼要使用 on。試比較下面的例句：
- At the Lunar New Year, the Lion Dance is one of the most popular traditional performances. (在農曆新年，舞獅是最受歡迎的傳統表演之一)
On Lunar New Year’s Day, we always play mahjong. (在農曆新年，我們都會打麻將) - We meet for lunch once in a while at/on the weekend. (我們週末偶爾會見面吃頓午餐)
The music festival is always held on the first weekend in February. (這個音樂節都在二月的第一個週末舉行)
In 還是 on 呢？
雖然我們使用 in the morning、in the afternoon、in the evening 和 in the night 來表示「(在) 上午」、「(在) 下午」和「(在) 晚上」，但若是指特定的上午、下午和晚上，則要使用 on。試比較下面的例句：
- The armed robbery took place at five in the morning. (這起武裝搶案發生在清晨五點)
His private jet took off on the morning of the tenth of December. (他的私人飛機在 12 月 10 日早上起飛) - He always gets home at seven in the evening. (他總是在晚上七點回到家)
I usually play basketball on Sunday evening. (我經常在星期天晚上打籃球) - In the afternoons, I like to listen to the radio. (每到下午，我喜歡收聽廣播)
Cindy always goes to see a movie on Saturday afternoons. (辛蒂總是在星期六下午去看電影)
At 還是 in 呢？
In the night 和 at night 都意為「(在) 夜晚」，但前者通常指特定的夜晚，而後者則泛指任何夜晚。例如：
- I woke up several times in the night. (我夜裡醒來好幾次)
- It’s not safe for a girl to walk alone at night. (女孩子夜晚踽踽獨行並不安全)
不使用 at、on 或 in 的時間詞
當時間詞的前面已有 each、every、next、last、some、this、that、one、any 和 all 等修飾語時，不可再使用 at、on 或 in。例如：
- She got married last Christmas. (她去年聖誕節結的婚) (正)
She got married at last Christmas. (誤) - Is John coming this weekend or next weekend? (約翰這個週末要來還是下個週末要來？) (正)
Is John coming at this weekend or next weekend? (誤) - I play baseball every Saturday. (我每週六打棒球) (正)
I play baseball on every Saturday. (誤) - Sarah is starting school next September. (莎拉明年九月開始上學) (正)
Sarah is starting school in next September. (誤)