Sy moes besig gewees het om te trek. vs. Sy moet besig wees om te trek.
Sy
3sg.f
moes
must.pst
besig
busy
ge-wees
ptcp-been
het
have.inf
om
inf.conj
te
inf
trek.
move.inf
vs.
vs.
Sy
She
moet
must
besig
busy
wees
be.inf
om
inf.conj
te
to
trek.
move.inf
She must have been in the process of moving. vs. She must be in the process of moving.
Comment:
INF.CONJ signals the infinitival conjunction, cognate with Dutch om and German um.
Besig wees om te is an aspectual phrase expressing the progressive (and not necessarily an agentive progressive - cf. Sy is besig om te sterf [3SG.F is busy INF.CONJ to die] 'She is dying').
Although Mood (moes/moet) precedes Aspect (besig (ge)wees om te), which in turn precedes Tense (het vs. zero), it is clear from the form of the auxiliaries that the actual sequence is Mood-Tense-Aspect: modals select for infinitives, which means that moes above selects het (the infinitive form of the PST auxiliary - cf. om te verstaan het [INF.CONJ to understand have] 'to have understood'), which in turn selects for the past participle gewees ('been').
The form of the Mood-expressing auxiliary is also determined by Tense: moet isn't standardly an option with het (some colloquial varieties permit Sy moet besig gewees het om te trek), while moes, under the same modal interpretation, isn't an option where het is absent (i.e. Sy moes besig wees om te trek can only mean 'She should have been busy moving (but wasn't)').