核心问题
They were expecting the surgeon to finish the operation ____ dawn. A. before B. till C. on D. in
介词 before 与介词 till 都表示“在……之前”，上道题除选 A (before) 外，能否还可选 B (till)?
词典解答
不能选 B(till)。因为介词 till(until)，如同连词 till(until)一样，在肯定句中，它只能与持续动词(durative verbs)，即持续一段时间的动词(如 be, live, like, work, camp 等)连用，而且表示“一直持续到……为止”。请对比下列例句：
-
My girlfriend worked there till Christmas. (CGEL)
-
They talked till dawn. (Lewis)
-
We must wait till Monday. (Gronin)
-
The ceremony will take place before the meeting. (CGEL)
-
I have to leave before midday. (CGEL)