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Author Topic: Modals are not meant for absolute certainties  (Read 4755 times)
Joe Carillo
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« on: May 04, 2013, 07:54:24 AM »

Here’s a fascinating question on modals from FH, an English teacher in Iran, that came by e-mail last May 2, 2013:

As you know, we use the structure “must have + past participle” when we are sure that an action happened in the past. For example, “I rang the bell several times, but they didn’t open the door. They must have gone out.”

Here’s my question: What’s the negative form of the structure “must have +past participle”? My friend says that when we are sure an action did NOT happen in the past, we should use the structure “can’t/couldn’t have + past participle.” For example, “Where is she? She couldn’t have gone out—the door’s locked.”

I disagree with my friend. I think when we’re sure that an action has NOT happened in the past, we should use the structure “must not have + past participle.” For example, “Where is she? She must not have gone out—the door’s locked.”

What do you think?

My reply to FH:

You and your friend are grammatically correct about the negative modal forms “must not have + past participle” and “couldn’t/ can’t have + past participle,” but both of you are fundamentally mistaken in thinking that they can be used to denote with certainty that an action didn’t happen in the past. On the contrary, these forms denote only a strong belief or conjecture that the action didn’t happen.

Remember now that the auxiliary verbs “can,” “could,” “must,” “might,” and “may” are modals that indicate conjecture, supposition, or belief rather than established facts or absolute certainty.

“Can” and “may” are often interchangeably used to denote possibility or permission, as in “She can go” or “She may go.” On the other hand, “could” is used as the past tense form of “can,” as in “We discovered she could sing”; for the past conditional, an in “She assured me that she would come if she could”; and as an alternative to “can” in suggesting less force or certainty, as in “I hope you both could come.” (In negative constructions, though, “may” is rarely used; instead of “mayn’t,” what’s usually used is “cannot” or “can’t,” as in “She can’t go” instead of “She mayn’t go.”)

“Must” is used to denote what can logically be inferred or supposed, as in “It must be risky to sail in such bad weather,” and “may” is used to indicate possibility or probability, as in “You may be right that he took the money.” “Might” is used to indicate a lower probability or possibility than “may,” as in “We might catch up with you if the rain stops,” and to express probability or possibility in the past, as in “She might have sold her car after all.” It is also used as a polite alternative to “may,” as in “Might I ask who’s on the line?” or as a polite alternative to “should,” as in “You might at least express appreciation for the favor.”

We can thus see that the positive modal forms “must have + past participle” and “could have + past participle” couldn’t be expressions of certainty at all. And neither could their negative modal forms “must not have + past participle” and “could not have + past participle” denote certainty that the action didn’t happen. They just express strong supposition or conjecture.

So, for actions that surely happened in the past, we absolutely can’t use the positive modal form “must have + past participle” as in this example of yours: “I rang the bell several times, but they didn’t open the door. They must have gone out.” Instead, we must establish the action in that second sentence as an objective fact: “I rang the bell several times, but they didn’t open the door. They surely had gone out” (or “They had gone out for sure.”)

For actions that surely didn’t happen, neither can we use the negative modal form “must not have + past participle” as in your friend’s example: “Where is she? She must not have gone out—the door’s locked.” We also must establish that the woman is indeed still inside the house: “Where is she? She surely had not gone out—the door’s locked” (or “She had not gone out for sure—the door’s locked.”)

In each case, the subject’s going out or not going out must be an absolute certainty.
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