You must be | It must be – English Grammar Profiler

In the English Grammar Profile, B1 point 69 in the category of MODALITY is defined:

DEDUCTIONS AND CONCLUSIONS: ‘must’ to guess and deduce, most commonly with ‘it must be’, or ‘you must be’.

*We cannot ensure automatically that this form will point to the usage.  We have deleted some ngrams that are obviously not the usage. Both examples in the EGP are followed by an adjective.  The surest of these are adjectives with negative meaning.

A search in iWeb for:

it|you must be _JJ

3 YOU MUST BE AWARE 2142 4 IT MUST BE TRUE 1798 5 YOU MUST BE SURE 1384 6 IT MUST BE GOOD 1096

7 IT MUST BE POSSIBLE 1094

8 IT MUST BE HARD 750

BBC George Floyd death: How US police are trying to win back trust

11 IT MUST BE LIKE 600

14 IT MUST BE DIFFICULT 539

15 YOU MUST BE FAMILIAR 530 16 IT MUST BE NICE 466 17 YOU MUST BE COMFORTABLE 439 18 YOU MUST BE ELIGIBLE 353

19 IT MUST BE EASY 347

21 IT MUST BE TOUGH 266

22 IT MUST BE UNIQUE 264 23 IT MUST BE RIGHT 260 24 IT MUST BE GREAT 259 25 YOU MUST BE CONFIDENT 243

26 IT MUST BE OK 223

27 IT MUST BE FRUSTRATING 200
28 YOU MUST BE CRAZY 193

29 IT MUST BE SAFE 162 30 IT MUST BE IMPORTANT 161 31 YOU MUST BE CERTAIN 156

32 YOU MUST BE NEW 148

37 IT MUST BE AVAILABLE 146
38 IT MUST BE OKAY 145

39 IT MUST BE BAD 145

40 IT MUST BE BETTER 137 41 IT MUST BE OBVIOUS 134

42 IT MUST BE FREE 124

43 IT MUST BE WRONG 118

45 YOU MUST BE HAPPY 107 46 IT MUST BE DIFFERENT 105

47 YOU MUST BE GOOD 104