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What are the names of these forms?
1)could + infinitive (es they could go)
2)could have + past participle of verb (ex. they could have worked)
3)could have been (they could have been working)
4)could be (they could be working)
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They are all modal forms because 'could' is a modal verb. "Modals" is the general name. Various books may have different names for the forms you cited. I am not aware of any universal standard set of terms for naming them.

I would use the following terms.

could/may/might/can/should/ ...

go modal
be going modal progressive
have gone modal perfect
have been going modal perfect progressive

Other names are:

could/may/might/can/should/ ...

go modal with infinitive
be going modal with progressive infinitive
have gone modal with perfect infinitive
have been going modal with perfect progressive infinitive
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As you see, I prefer to use the less wordy descriptors.
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The term 'continuous' substitutes for 'progressive' in some textbooks.

CJ