英语语法：225 Is It OK to Switch Verb Tenses?

Today’s topic is whether it’s OK to switch verb tenses in conditional¹ sentences. It all started with a grammar question about “was” and “were.” On the Grammar Girl Facebook page, Veronica wanted to know whether she should write “If the test was readministered,” or “If the test were readministered.”
What Is the Subjunctive Mood?
Ellen responded that it should be “If the test WERE readministered,” noting that “were” is the form of “be” you use when you’re talking about a hypothetical situation. That is true, at least if you’re talking about present or future time. If it’s unlikely² that the test will be readministered, but you’re considering that remote possibility, you’d say “if the test WERE readministered.” It's called using the subjunctive mood, and we covered it back in episode³ 160: Subjunctive Verbs.
On the Facebook discussion⁴, I added one clarification⁵: that you could also say “If the test WAS readministered” if you’re talking about something in the past that actually might have happened. The example I gave was, “If the test WAS readministered Tuesday, the answer key WILL BE locked in Professor Hilda’s room. Otherwise, Professor Stockton will have it.”
Is It OK to Switch Verb Tenses?
But my example opened up another issue for Luther, also known as the Grammar Geek, who wrote on his blog, “I am instantly⁶ drawn⁷ to the mismatch between ‘was’ and ‘will be,’ which breaks my first commandment of grammar. ‘Thou shalt not mix ... verb tenses.’” He argued that because the verb in the “if”-clause⁸ is in the past tense (If the test WAS readministered), the main clause should be in the past tense: “the answer key WOULD be locked in Professor Hilda’s room,” not “the answer key WILL be locked in Professor Hilda’s room.”
I was pretty sure it was OK to switch tenses, but there was a chance I was wrong, and Luther is right that switching verb tenses is something to be careful about, so I asked my guest-writer Neal Whitman, who has a PhD in linguistics⁹, to look into the question.
When Switching Tenses is OK
Neal says we do need to switch tenses sometimes. Otherwise, there’d be no need for English to have different tenses at all! (In fact, some languages don’t have tenses, but that’s a different story.) There's more to say about switching verb tenses than we have time for in this podcast, so we'll stick to the case that prompted¹⁰ Luther's comment--verb tenses in conditional sentences--and continue to use Veronica's sentence about the test as an example.
Let’s suppose the test really may have been readministered on Tuesday, and we begin the sentence, “If the test was readministered on Tuesday.” We used “was” because it's possible that it really happened. So far, so good.
Next, what tense do we use in the main clause that follows? Well, it depends on whether we’re saying something about the past, the present, or the future. In other words, we choose our tense the same way we usually do. Simple, right?
We might say
If the test was readministered on Tuesday, then it WAS Professor Hilda who proctored it.
Or, we might say
If the test was readministered on Tuesday, then the grades ARE now in Professor Hilda’s office. (Or even “the grades WILL be in her office now.”)
Or, we might say
If the test was readministered on Tuesday, we WILL be telling the students their grades next week.
When to Use the Past Perfect Tense
Now let’s see what happens when the “if” clause refers to an unreal situation. Suppose we know the test WASN’T readministered on Tuesday. In that case, we would phrase the “if”-clause like this: “If the test HAD BEEN readministered on Tuesday.”
“Had been readministered” is in the past perfect tense—the tense that uses the past-tense form of the helping¹¹ verb “have.” One of the uses for the past perfect tense is to talk about unreal past-time situations.
What about the verb tense in the main clause that follows? Once again, it depends on whether we’re saying something about the past, present, or future. But this time, since we’re talking about unreal situations, we use verb forms with the helping verb “would,” “could,” or “might.”
So we might say
If the test had been readministered on Tuesday, Professor Hilda WOULD HAVE proctored it.
“Would have” shows that this is a past-time unreal event we’re talking about. Professor Hilda DIDN’T proctor the exam; someone else did.
We could also say
If the test had been readministered on Tuesday, then the grades WOULD BE in Professor Hilda’s office now.
In this sentence, “would” refers to a present-time situation that isn’t actually true. You could imagine the speaker going on to say, “But they’re NOT here! So the test must have been readministered some other day.”
And finally, we could even say
If the test had been readministered on Tuesday, then we WOULD TELL students their grades next week.
In this sentence, “would” refers to an unlikely or impossible future event, namely¹², telling the students their grades next week. You can imagine the speaker continuing, “But since it wasn’t readministered until Thursday, we won’t be able to tell the students their grades until after the break!”
Grammar Girl’s Switching Tense Bottom Line
So to sum up, we started off with an “if” clause referring to a past-time event: the readministering of a test. The verb in the “if” clause had a different form depending on whether the event really could have happened or it probably didn’t happen. But either way, the verb in the “if” clause didn’t put any tense requirements on the verb in the main clause. We chose the tense in the main clause the same way we always do: depending on whether we’re talking about a situation in the past, the present, or the future.
The same rules apply if the “if” clause is about a present or future event, too, regardless of whether it could actually happen, or is unlikely or impossible. We could begin with “If the test IS readministered” if that’s likely to happen, or with “If the test WERE readministered” if it’s unlikely, but either way, the only thing that matters for the verb tense in the main clause is whether it’s about something in the past, the present, or the future.
Thanks to Luther for bringing up such an interesting question. You can find his blog at thegrammargeeksez.blogspot.com, and join the lively discussion at the Facebook Grammar Girl page at Facebook.com/GrammarGirl.
 

1 conditional   

adj.条件的，带有条件的

参考例句：

• My agreement is conditional on your help.你肯帮助我才同意。
• There are two forms of most-favored-nation treatment:conditional and unconditional.最惠国待遇有两种形式:有条件的和无条件的。

2 unlikely   

adj.未必的，多半不可能的；不大可能发生的

参考例句：

• It was very unlikely that he would do that.他不见得会做那种事。
• It is unlikely that she will come. 她不大可能来了。

3 episode   

n.（作品的一段）情节，插曲，系列事件中之一

参考例句：

• The episode was a huge embarrassment for all concerned.这段小插曲令所有有关人员都感到非常尴尬。
• This episode remains sharply engraved on my mind.这段经历至今仍深深地铭刻在我的心中。

4 discussion   

n.讨论，谈论；论述

参考例句：

• It is certain he will come to the discussion.他肯定会来参加讨论。
• After months of discussion,a peace agreement is gradually taking shape.经过几个月的商讨，和平协议渐渐有了眉目。

5 clarification   

n.（液体的）澄清；净化；澄清法；（意义等的）澄清、说明

参考例句：

• Employers are seeking further clarification of the proposals. 雇主们正在寻求进一步澄清这些提案。 来自《简明英汉词典》
• The whole issue needs clarification. 整个问题都需要澄清。 来自辞典例句

6 instantly   

adv.立即，立刻

参考例句：

• She was used to having her orders instantly obeyed.她习惯了让人即刻服从她的命令。
• Though he slept soundly,he awoke instantly.他虽然睡得很香，但是马上就醒了。

7 drawn   

v.拖，拉，拔出；adj.憔悴的，紧张的

参考例句：

• All the characters in the story are drawn from life.故事中的所有人物都取材于生活。
• Her gaze was drawn irresistibly to the scene outside.她的目光禁不住被外面的风景所吸引。

8 clause   

n.（正式文件或法律文件的）条款，从句，分句

参考例句：

• The sentence consists of a main clause and a subordinate clause.这句中有一个主句和一个从句。
• What clause do you require in the contract?你要求在合同中订上什么条款？

9 linguistics   

n.语言学

参考例句：

• She plans to take a course in applied linguistics.她打算学习应用语言学课程。
• Linguistics is a scientific study of the property of language.语言学是指对语言的性质所作的系统研究。

10 prompted   

v.为（演员）提示台词( prompt的过去式和过去分词 )；促使；为（发言者）提示；导致

参考例句：

• His evasive reply prompted me to ask another question. 他的含糊其辞的答复促使我又提了一个问题。 来自《简明英汉词典》
• The sight of the ships prompted thoughts of his distant home. 看到船使他想起遥远的家乡。 来自《简明英汉词典》

11 helping   

n.食物的一份&adj.帮助人的，辅助的

参考例句：

• The poor children regularly pony up for a second helping of my hamburger. 那些可怜的孩子们总是要求我把我的汉堡包再给他们一份。
• By doing this, they may at times be helping to restore competition. 这样一来, 他在某些时候,有助于竞争的加强。

12 namely   

adv.即，也就是

参考例句：

• Only one boy was absent, namely Harry.只有一个孩子缺席，就是哈利。
• Only one person can answer the question namely you.只有一个人能回答这个问题，那就是你。