Could you tell me the reason why the bare infinitive part is "be going(progressive form?), not just "going"? Are there any differences between "had better be going" and "had better going"? Thanks in advance!
Well, in that case we’d better be going.
'going' is an -ing form (participle/gerund) not an infinitive. continuous verb form. I am (present) going = present progressive/continuous of GO I have been (present perfect) going = present perfect progressive/continuous of GO be (infinitive) going = progressive/continuous infinitive of GO I had better go (non-progressive/continuous) and I had better be going (progressive/continuous) are possible.
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PonyFannuanceThey are essentially the same, but if you want nuances, the 'be going' version suggests starting to think about leaving, maybe making some preliminary moves to take leave, as from a party among friends. The 'go' version suggests something more immediate, even a quick escape, as when you notice signs of an imminent barroom brawl.