Both 'must' and 'have to' can realise high value obligation or probability, the latter form evolving from an earlier conative function; see Halliday & Matthiessen (2014: 145, 572-3, 583).
modulation: obligation (high value)
she | must | go |
Theme | Rheme | |
Actor | Process: material | |
Subject | Finite | Predicator |
Mood | Residue | |
she | has | to go |
Theme | Rheme | |
Actor | Process: material | |
Subject | Finite | Predicator |
Mood | Residue | |
modalisation: probability (high value)
she | must | have gone |
Theme | Rheme | |
Actor | Process: material | |
Subject | Finite | Predicator |
Mood | Residue | |
she | has | to have gone |
Theme | Rheme | |
Actor | Process: material | |
Subject | Finite | Predicator |
Mood | Residue | |
