IS用于回答别人问时间,如:--What time is it?---It is 8 o'clock.
at用来表达做某事的时间.如：I usually go to bed at 10 o'clock in the evening.
a点b分(不超过30分)用b past a.如:10点15分fifteen past ten.
差a分就b点用 a to b.如：差8分11点eight to eleven.
quarter用来表示时间是“刻（1相当于15分钟）”.如：a quarter to twelve(差1刻就12点)
“我9点50吃午饭”是 I have lunch at ten to ten.