Question

for past can we use"" i must be"​

Answer 1

Yes we can... Only difference, instead of " I must be", you have to use " I must have" or "I must have had -"

For example-

I must have forgotten my keys in the car.

Here, forgotten is the past participle of the word forgot and hence it is in past tense

She must have had some problem.

If you have to use i must be, your sentence will be in present tense.

Hope this helps

Have a great week ahead

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Answer 2

Answer:

yes, but we say in past tense

I must have, I must had