Descriptive grammatical term sought

Hello everyone

My sister is studying for a TEFL qualification.
One of the questions she got this week referred to these structures:

4) It's a really bad picture.
i) Oh, it must be the aerial.
ii) Oh, it might be the aerial.

I only have Oxford Guide to English Grammar & the book by Swan (Practical English Usage) - neither of which explicitly state the descriptive terms for the structures "it must be" & "it might be". I am sure that there are many of you here who know the names of these structures: please let me know (I would also appreciate it if you told me what sources you use).

Thank you
Jan

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