"Must" and "cannot" with "be going to"

Question

My father and I have been discussing whether it is possible to concatenate the semi-modal verb "be going to" with modal verbs "must" or "cannot". In our opinion, which we both agreed on, this combination is likely possible:

1. She must be going to tell him the truth. - meaning "There is a high probabily that she is going to tell him the truth"
2. He can't be going to skip classes today. - meaning "I don't believe that he is going to skip classes today"

What do you native speakers think of this?

Answer 1

Personally, I have trouble reading "be going to" as semi-modal in those sentences, and a cursory Google search seems to confirm my suspicions that for most native speakers, it cannot be semi-modal in that context either.

My initial reading of:

She must be going to tell him the truth.

is that "She must be going to where he is to tell him the truth". There is a similar assumption with your second example that the going is an actual action, and that "he" must be physically going to school/university (or any other place with classes).

That said, certain American native speakers allow double modals, and I would not be surprised if your examples are considered colloquial or "bad grammar" by them, but nonetheless something they'd use informally. See Yale's page on multiple modals for a formal analysis and examples:

I don't think I have any grants you might could apply for.

In this case, the might could is roughly equivalent to might be able to. Your examples would then mean what you expected them to mean.

Answer 2

If I intend to tell her the truth, I can say:

I am going to tell her the truth.

No change of physical location is intended there. It is a future.

But locomotion might be implied by the same structure:

I am going to feed the parking meter.

I could mean there that I'm leaving to put money in the meter.

The two verb phrases would have slightly different intonations.

I'm going to téll her the truth.

I'm góing to feed the parking meter.

Must be going to VERB expresses a predicted future.

He must be going to tell her the truth.

sounds like someone predicting, with some degree of certainty based on evidence, what a character in a soap opera, say, intends to do.

These are perfectly legit mainstream constructions, not considered regional or substandard in any way. They're not like "might could". can|must|could|should|might|may be going are not double modals.