有两位读者来信问到as if从句的谓语动词应用什么时态的问题。一位读者说有这样一道选择题：
(1)He knew nothing about the matter, but he talked as if he_________it.
A. knows B. knew C. had known D. would know
请问应选B还是C?
另一位读者说，有这样一句话：
(2)Holding his head high, he walked past the pole and the soldiers as if they didn't exist.
请问这里的as if从句为什么不用与过去事实相反的过去完成时had not existed?过去一般虚拟式不是表与现在事实相反吗？
首先必须指出，上述句（1）与句（2）as if从句中的过去一般时knew和didn't exist不是直陈语气，而是虚拟语气。如何解释这种过去一般虚拟式呢？ 有一位外国学者说这种过去一般虚拟式是作者当时（写作时）的一种想象，他将他当时的想象用于过去的叙述中了。我们觉得这一看法是合理的。
关于as if从句的谓语动词应用什么时态，在中国学生中，可能是一个带有普遍性的问题。
首先有一个as if从句应用什么语气的问题，曾有语法家认为as if之后必须用虚拟时态。当今这种抱残守缺的人，即使有，恐怕也不多了。绝大多数语法家都认为，as if之后既可用虚拟语气，亦可用直陈语气。而语言实际也是如此。虚拟语气表与事实相反或很可能相反，直陈语气则表有一定的现实性。
在未谈用什么语气之前，必须指出，as if从句所用的时态，与主句所用的时态是没有什么必然联系的。换言之，不管主句用什么时态，现在时态也好，将来时态也好，过去时态也好，都不会对as if从句的时态产生什么影响。例如：
(3)He looks as if he were ill.
(4)He will look as if he were ill.
(5)He looked as if he were ill.
(6)He had looked as if he were ill.
以上四例的主句用了四个不同的时态，as if从句则可以不变应万变，皆用were。这是为什么呢？ 答案是：as if从句所表示的比喻或想象，完全是说（写）话人在说（写）话时所作的比喻或想象，与主句所表的情景（现在、过去或将来的）无关。
现在让我们开始探讨一下as if从句应用什么时态，先从虚拟时态谈起吧。
as if从句最常用的虚拟时态就是过去一般时。这种时态表与现在的事实相反，也就是表说话人当时所认为的与事实相反，而这也就是为什么上述句（1）应用knew和句（2）用didn't exist的缘故。上述句（3）、（4）、（5）、（6）中的were也都属于这种用法。（were显然是虚拟时态；而knew与didn't exist虽与直陈语气的过去一般时的形式相同，但从上下文看，二者也是虚拟时态）虚拟时态过去一般时所表的动作或状态与主句的谓语动词，是同时发生或存在的。
as if从句也常用虚拟时态过去完成时，但它并不表与过去事实相反，而是多表动作或状态的完成，相当于汉语的“已经，曾经，……过”等。所以它所表的动作或状态是先于主句的谓语动词发生或存在的。例如：
(7)He looks as if he had been ill. 他（现在）看上去像是病过一场似的。
(8)He looked as if he had been ill. 他（过去某一时间）看上去像是病过一场似的。
从以上两个例证可以看出，as if从句中的过去完成时，和过去一般时一样，也不受主句的动词时态的影响。再如：
(9)He looks （或looked）as if he had seen a ghost.
(10)He has （或had）known me long, but he treats （或treated）me as if he had never known me.
(11)People were scattering as if a rain had begun to fall in the room.
(12)It isn't as if I'd given him to believe that I was experienced! I made it clear that I wasn't. （It isn't as if I'd given him...was experienced是一感叹句，其意为“我压根儿就没有叫他信以为我富有经验!”）
请注意有时有人用过去完成时代替过去一般时。有人说这种过去完成时具有意外或惊异等感情色彩，如：
(13)He treated me as if I had been a stranger.
(14)I took him by the waist as if he had been a sack of bran.
不消说，上述两例中的had been皆可代之以were。
as if从句也可用过去将来一般时would+动词原形，其所表的动作或状态自然发生或存在于主句的谓语动词之后，例如：
(15)It seems as if the boat would sink .
(16)It seemed as if the boat would sink . （在一定的上下文中，would sink也可能是直陈语气）
(17)He looks as if butter would not melt in his mouth.
as if从句亦可用上述三种虚拟时态的进行式(were doing, had been doing, would be doing)，这里就恕不举例了。
as if从句也可用直陈语气的各种时态。这种时态表有一定的现实性，并不与事实相反。当今在英语中，这种用法已可谓屡见不鲜，下面只举几例即可见一斑。
(18)He acts as if he is in love with her. （现在一般时）
(19)He looked as if he was ill. （过去一般时）
(20)She looks as if she will be like you when she grows up. （将来一般时）
(21)It looks as if it is going to snow. （直陈语气表将来）
关于as if从句的谓语动词的时态，就谈这么多吧。
