Halliday & Matthiessen (2014: 568):
The primary group (α) may be finite or non-finite; it is the primary group that carries the mood of the clause, e.g. she tried to do it, what was she trying to do, was she trying to do it, try to do it, having tried to do it etc. The secondary group (β γ δ ...) is always non-finite, this being the realisation of its dependent status.