Adopting corpus analytical procedures, we identified two major must constructions, [must+be] and [must+"ve/have], whose central member [there+must+be+some] conducts the topic extending function while [she+must+"ve/have+been] is related to the speaker"s evaluation of the condition of an individual identified as she.
it is also found that when the construction appears at the beginning of an utterance, it is often preceded by markers like but, mm, well, er, yes, or yeah, which indicates the speaker"s readiness to launch a negative comment or counter argument.