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The past of must is must have (which is used with the past
participle of a verb). Must have (which is often contracted
to must've in speaking) is used to show a conclusion about
the past. The negative form of must have is must not have.
Must and not are not contracted for a past negative conclusion,
but in speaking, a contraction is sometimes made with not
and have.
Examples:
| A: |
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Where's Bob? He said that he'd be here at 9:00,
and now it's after 9:30. |
| B: |
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He must've had some kind of problem. He's
normally very punctual. |
(Because B knows that Bob is normally very punctual instead
of being late, B concludes that Bob had some kind of problem.)
| A: |
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What happened to Jill? She got a phone call and
then she started crying. |
| B: |
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She must've received some bad news. |
(B concludes that Jill received some bad news during the call
because she started crying just after she received it.)
| A: |
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Why isn't Joe here? I sent him e-mail saying that
our committee would meet at 9:00. |
| B: |
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He must not have (must not've) read his e-mail.
He's very conscientious about attending meetings. |
(Because B knows that Joe is very conscientious about attending
meetings, and because A told Joe about the meeting in an
e-mail message, B concludes that Joe didn't read his e-mail.)
| A: |
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I saw Alice and waved to her, but she didn't
wave back. She just kept on walking. |
| B: |
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She must not have (must not've) seen you.
She's a very friendly person. |
(B knows that Alice is a very friendly person, so when B
hears that A waved to Alice and Alice just kept on walking,
B concludes that Alice didn't see A.)
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Special Notes:
| 1. |
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Because 've sounds very much like of, native speakers
sometimes write *must of when must've is actually
the correct form. |
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| 2. |
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In fast, casual speech, the 've in must've often changes
to a sound that is easier to pronounce. As a result,
must've often sounds something like "musta." |
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| 3. |
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In fast, casual speech, must not've also changes to
sounds that are easier to pronounce. As a result,
must not've often sounds something like "muss nodda." |
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| 4. |
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Must have shows a conclusion about the past, but not
a past requirement. To show a past requirement, had to
is used:
Bob couldn't join us. He had to work.
I'm sorry, but Ms. Keller isn't in today. She had to
go out of town on business.
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| 5. |
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The negative form of had to is didn't have to, but it
doesn't show a negative past requirement. Instead, it
shows that something wasn't necessary:
Susie didn't have to work last night. (It wasn't necessary
for Susie to work last night.)
You didn't have to do that. (It wasn't necessary for
you to do that.)
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| 6. |
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Negative past requirements cannot be shown with must
or with had to. Instead, complicated restatements such
as the following must be used:
It was necessary for them not to leave the building.
They were required not to leave the building.
It was necessary for him not to eat or drink anything
before his medical tests.
He was required not to eat or drink anything before
his medical tests.
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