Not "Our class is on seven o'clock." — say "Our class is at seven o'clock.". 钟点用 at，不用 "on"。
试一试: Ethan: Come early. The center is open ___ eight-thirty. (at / on / in) 答案: at — Eight-thirty 是钟点，钟点用 at。"On" 接某一天，"in" 接一天中的某个时段。
on = 某一天
记住这个搭配：on + 星期几。
We have class on Monday.
The center is closed on Sunday.
I see my friends on the weekend.
Not "We have class at Monday." — say "We have class on Monday.". 某一天用 on，不用 "at"。
试一试: Maya: Bring your book. The art class is ___ Friday. (on / at / in) 答案: on — Friday 是一周里的某一天，说某一天要用 on。"At" 接钟点，"in" 接一天中的某个时段。
in the morning，但 at night
记住这些：in the morning / afternoon / evening，但要说 at night。
I study in the morning.
We have class in the afternoon.
The center is closed at night.
Not "The center is closed in night." — say "The center is closed at night.". 我们说 in the morning / afternoon / evening，但说夜里要用 at night。
试一试: Maya: I draw my pictures ___ night. It is quiet then. (at / in / on) 答案: at — Night 是一天中唯一用 at 的时段。morning、afternoon 和 evening 都用 in。
When is the class?
问时间，就用 When is 开头。回答可以把星期几和钟点一起说出来。
When is the class? — On Monday, at seven.
When is your class? — On Saturday, at noon.
When is the next one? — In the afternoon.
Not "When are the class?" — say "When is the class?". 一节课是一件事，所以问句用 is：When is the class?
试一试: Ethan: ___ Maya: On Wednesday, at a quarter after ten. 选出 Ethan 的问题。 (When is the class? / Where is the class? / Who is the class?) 答案: When is the class? — Maya 回答的是星期几和钟点，所以 Ethan 问的是 When is the class? 问 "Where" 会得到一个地点，问 "who" 会得到一个人。
试一试: Ethan: Come after the last class. We are free ___ the evening. (in / at / on) 答案: in — evening 是一天中的一个时段，这类词用 in。"At" 适合 night 或钟点，"on" 适合 Sunday 这样的某一天。
常见错误有哪些？
错误
正确
原因
Our class is on seven o'clock.
Our class is at seven o'clock.
钟点用 at，不用 "on"。
We have class at Monday.
We have class on Monday.
某一天用 on，不用 "at"。
The center is closed in night.
The center is closed at night.
我们说 in the morning / afternoon / evening，但说夜里要用 at night。