at, on, in(A) at表示“在某一时刻，某一时点”。如：I get up at 6:00 every day. 我每天6点起床。注：at daybreak在黎明；at noon在中午；at night在夜间；at Christmas在圣诞节期间；at the age of five在五岁时。(B) on表示“在具体某或某天的上、下午”。如：I heard a shot on (the morning of ) March 18. 三月十八日(早晨) 我听到一声枪响。We don’t have classes on Sunday. 星期天我们不上课。(C) in表示“在某世纪、年、季度、月、周”以及泛指的上、下午、晚上。如：in the 20th century在20世纪；in 1999在1999年；in winter在冬季；in September在九月；in the morning/afternoon/evening 在上午/下午/晚上。
“a portion of , a series of, a kind of, a body of, a species of, a pair of+名词”作主语时，其谓语动词一般用单数形式。【例如】A series of accidents has occurred recently in America.A pair of new glasses is quite expensive.事件、机构、国名、作品等专有名词作主语时，谓语动词用单数。【例如】The Adventures of Huckleberry Finn was written by Mark Twain and it is an interesting book.The United Nations is an organization to defend world peace.</p 产品：澳博培训供货总量：不限产品价格：议定包装规格：不限物流说明：货运及物流交货说明：按订单
