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I had a sentence transformation exercise and I had to say that sentence "I'm sure it WAS Tom who cleared everything up." with (It must...) and I wrote "It must was john who cleared up everything" but it's said it's wrong and the correct answer is "It must HAVE BEEN Tom who cleared everything up.". I don't think there's a mistake even if I write "It must BE John who cleared up everything.". So, Am I wrong and why?

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    must was is ungrammatical. You probably meant to say it must've been Tom who cleared up everything. - must be indicates certainty in the present, must have indicates certainty in the past, that is, when you are sure about something that happened in the past.
    – Schwale
    Commented Jun 19, 2016 at 20:00
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    Yes, you are wrong. Modal verbs like "must" can only be followed by an infinitival verb phrase. So to transform I'm sure it was Tom ... by using "must", you have to say It must have been Tom ..., where "have" is an infinitival verb-form
    – BillJ
    Commented Jun 19, 2016 at 20:03

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Because only a bare infinitive, such as be or have, can follow a modal verb such as must.

The have in must have been John is a bare infinitive. It is not a conjugated verb. There is no

*It must has been John who...

any more than there is a

*It must was John who...

Both those constructions are wrong, and I marked them with an asterisk (*) to show that they are ungrammatical.

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