下面是一道多项选择题：
He will be back ____ three o'clock.
A. in B. after C. for D. on
我们班同学对于选 A(in) 还是选 B(after) 发生了争论。请指教。
在表示以现在为起点的“在……之后”这个含义时，介词in之后应接时间段(a period of time)，介词after之后则应接时间点(a point of time)。例如：
-
It's two o'clock. I'll come in an hour( = at three o'clock). (LDCE) 现在是两点,我一小时后回来(即三点钟回来)。
-
I shall come round in a day or two. (Voynich) 我一两天之后回来。
-
He'll come back after seven o'clock. 他将在七点钟之后回来。
但是，当以某个过去的时间为起点时，则 after 之后既可接时间点，也可接时间段：
-
Just after seven the rain began to fall. (Du Maurier) 刚过七点,就开始下雨了。
-
After a year and half of war Russia was badly defeated. (Hazen) 战争过了一年半,俄国被打得惨败。
-
After seven days of this I went to a party. (NCE) 这样过了七天之后,我去参加一个聚会。
不难看出,上道多项选择题应当选 B(after)。
有了上面的知识,再来做下面的题,就轻而易举了:
-
He will graduate ____ three years. A. after B. in C. for D. from
-
His mother will come back ____ four o'clock. A. in B. on C. after D. to
-
The fall semester begins ____ September 1. A. at B. in C. after D. during(TOEFL)
-
The doctor will be free ____. A. 10 minutes later B. after 10 minutes C. in 10 minutes D. 10 minutes after(全国高考,92-20)