Let us take the verb 'get', we can say both:
1- Someone gets to take something
2- Someone gets someone to take something
In the 1st sentence, 'get' is a modal morpheme, but in the other sentence 'get' is a causative morpheme. This case is not only found in English, in Riffian there is also this confusion:
1- i-ga ad i-ksi ci (he-did FUT he-take something)
2- i-ga x-k ad t-ksi-d ci (he-did on-you FUT you-take something)
How is it possible that a morpheme can be either causative or modal ? Can we speak of "ambi-auxiliarity" ?
X
get toVP
meansX
is allowed toVP
, so you can think of it as having a deontic modal sense (no epistemic sense, though, so it's not a full modal). On the other hand,X
getY
toVP
means thatX
causesY
toVP
, so you can think of it as being causative. Get is, after all, the causative/inchoative form of both be -- He got married -- and have -- He got a cold. Idioms have special affordances and grammar; it doesn't have to much to with get as with idioms.