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I suppose the modal verb 'must' works like below:

  1. I am sure that he is busy. → He must be busy.
  2. I am sure that he was busy. → He must have been busy.

I want to know how 'must' itself changes in past tense like this:

  1. I was sure that he was busy. → ?
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    Are you asking about "I must do this now" vs "I * had to* do that yesterday"? – Jim Feb 25 '15 at 3:04
  • Yes, I want to know how a sentence with 'must' changes into past tense. And I think 'had to' is not appropriate in this case... or is it? – Thunderweb Feb 25 '15 at 4:12
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As the past tense of "must" is also "must" (identical in form) the modal must is normally used only in present tense to avoid ambiguity. When you need the past tense you have to use forms of substitution verbs.

"must2" (the past tense form) can be used only in very limited cases. It may occur sometimes in written language in subordinate clauses. I think learners should not try to handle must2.

In most cases the modal must is given only with one form (present tense).

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As rogermue points out "must" is the same for both past and present. To indicate something happened in the past, you would just add additional information that contains a recognizable time frame.

I was sure that he was busy. → I thought he must have been busy

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  • What's about combination of verbs? "He must have done it" works? – Judicious Allure Jun 9 '19 at 19:58
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I must do it(present tense) I had to/was to do it(past tense)

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