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The task is to complete the second sentence so that it has a similar meaning to the first
sentence, using the word given.

The thief ran right past you so I'm sure you saw his face.

must

The thief ran right past you so ....................................... his face.

My answer:
The thief ran right past you so must saw his face.

Correct answer:
The thief ran right past you so must have seen his face.

I admit that it sounds better with present perfect but the original sentence is in the past.
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Yes, but "unspecified time before now" is a kind of "chewing gum" period extending from say your birth day until today. Since you cannot pinpoint the EXACT moment in te past when the action took place, you can't use a simple past. THAT's when you use the present perfect.
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Rule: Modal verbs (and "must" is one of them) cannot be followed by a past tense form. To express past tense after a modal add "have" and the past participle. This is not truly a "present perfect" but a "modal perfect". In the case of the modal perfect construction the difference between simple past and present perfect is neutralized.

This rule seems rather simple but important. I have to keep it in mind.

He must have lived here for five years. / He has lived here for five years.

I must have met him five years ago. / I met him five years ago.

paco
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Comments  
Rule: Modal verbs (and "must" is one of them) cannot be followed by a past tense form. To express past tense after a modal add "have" and the past participle. This is not truly a "present perfect" but a "modal perfect". In the case of the modal perfect construction the difference between simple past and present perfect is neutralized.

That is, adding "must" either to a simple past or to a present perfect creates the same modal perfect structure:

He read the book yesterday. > He must have read the book yesterday.
He has read the book twice. > He must have read the book twice.

CJ
Quote:
"We use the Present Perfect to say that an action happened at an unspecified time BEFORE NOW. The exact time is not important."
----------"before now" = "past" ( to me, anyway)

http://www.englishpage.com/verbpage/presentperfect.html
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Paco,

Your illustrations are 'simply' 'perfect' in every detail! Emotion: smile

Jim