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

（单词翻译）

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.