请看题：

―Excuse me, sir. Smoking is not allowed here.

―Oh, sorry I __________.

A. don’t know                                   B. didn’t know

C. haven’t known                              D. can’t know

【分析】此题应选 B。这是一道很好的测试时态的语境题。此题的情景是：一方提醒另一方不准抽烟，对方表示歉意并说自己不知道。这里的“不知道”显然是指对方“提醒”之前“不知道”(所以用过去时态)；假若用一般现在时，则指在对方提醒之后还仍然不知道，这不是很荒唐吗?

在做时态试题时，要特别注意上下文的语境：

1. ―We could have walked to the station; it was

so near.

―Yes. A taxi ________ at all necessary.

A. wasn’t                                          B. hadn’t been

C. wouldn’t be                                   D. won’t be

2. ―Your phone number again? I ________ quite catch it.

―It’s 7226109.

A. didn’t                                            B. couldn’t

C. don’t                                             D. can’t

3. ―Can I help you，sir?

―Yes. I bought this radio here yesterday，but

it _________ work.

A. didn’t                                            B. won’t    

C. can’t                                             D. doesn’t

4. ―Alice，why didn’t you come yesterday?

―I _________，but I had an unexpected visitor.

A. had                                               B. would            

C. was going to                                  D. did

答案：1. A 2. A 3. D 4. C