'Must' & 'Have To': Obligation & Probability

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