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此題應填unless還是until

請看下面這道題：

I didn’t manage to do it __________ you had explained how.

A. unless                        B. until                        C. when                        D. since

此題應選B。這道題從句意來看似乎選A，B都行。不少考生看不出選項A錯在什么地方，這里我們可以從句意和時態(tài)兩方面來分析：

選A，句意為：你若沒給我解釋，我就不會做好它。—-按此選擇，主句謂語應用would have managed之類的，也就是說這是一個虛擬語氣的句子。

選B，句意為：直到你給我作了解釋，我才會做。—-按此選擇，你先給我解釋，我后才會做，所以“解釋”用過去完成時，“會做”用一般過去，順理成章。

請做以下試題：

1. I didn’t know anything about it___ he told me.

A. unless                       B. until                          C. when                         D. since

2. _________ he spoke I hadn’t realized he was a foreigner.

A. Unless                       B. Until                         C. When                         D. Since

3. I didn’t realize he was our teacher _________ he came in front of me.

A. unless                       B. until                          C. when                          D. since

4. _________ he told me，I had no idea of it.

A. unless                       B. until                          C. when                          D. since

答案：1. B 2. B 3. B 4. B