Is 'had' in 'They had to be happy' used as a verb or adjective? [closed]

Solution 1:

Have to, has to, had to, having to, followed by an infinitive verb phrase, is an idiom. A verbal idiom -- it's a verb, to answer the question. A Modal verbal idiom. This idiom comes with its own special weird pronunciation rules (has to is /'hæstu/ -- never /z/, have to is /'hæftu/ -- never /v/), and its own weird grammar and meanings, like all modals.

It means the same as the modal auxiliary verb must, in both its Deontic (social, obligation) and its Epistemic (logical, conclusion) senses. (all modals have both kinds of meaning; it's a modal complication)

• She must be home by midnight. = She has to be home by midnight. (Deontic)
• This must be the right place. = This has to be the right place. (Epistemic)

So, the example sentence

• They had to be happy

is an epistemic use of the idiom (a periphrastic modal construction, to give it its full name), since They can't have an obligation to be happy in the past. Indeed, one can't have any obligation to be happy, even in the present; emotions are not subject to deontic modality. One can only be obliged to do the things one is capable of doing. And no one can be obliged to change the past.

The sentence means, then, that someone (perhaps the speaker) has concluded (from some unstated evidence) that They were happy, at the past time cited. It means the same thing as (but doesn't have the same structure as)

• They must have been happy.

That's also an epistemic conclusion, and also uses an auxiliary have, but it's not the same as have to. For one thing, the have in must have is pronounced /v/, not /f/. For another, must have is followed by a participle, not a to-infinitive. So it's different. It has to be, because had to is past tense, and must lacks a past tense. You can't say

• *They musted be happy

But you can say They had to be happy, meaning the same thing. The reason why there are periphrastic modal constructions like have to, ought to, be able to, be willing to, and the rest is that they have tensed forms and infinitives and participles like real verbs, but modals are all stripped down to bare metal and don't have any of the fixings needed for that. So we have both kinds and if one doesn't serve, we use the other. Language is like that.