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How are we to know whether the modal perfect is referring to the past perfect or the present perfect since the auxialiary always used after a modal is 'have'? [:^)]

Or does it not refer to a perfect tense/aspect at all?

Thanks
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Comments  (Page 2) 
English 1b3But sometimes modals do express time, tense, correct?
Yes, but often the expression of time is vague or not the main reason for using the modal verb. For example, should gives advice. There's no point in giving advice about a situation that's already occurred in the past. So should can only indicate present time, not past. As I have pointed out before, each modal verb has its own peculiarities of usage and meaning, so each has to be studied and mastered separately.

CJ
CalifJimSo should can only indicate present time, not past.
I assume you are not including the pefect modal tense: should have.

Thanks CJ. I grasp this subject far better now. (If you have an informative site on modal tenses etc, I'd appreciate the link.)
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English 1b3I assume you are not including the pefect modal tense: should have.
Yes. Your assumption is correct. You use should have when you're talking about advice that you would have given but is now too late to give. For example, You should have started earlier says that I would have advised you to start earlier, but you've already started at the time you did, so there's nothing that can be done about it now.

Google English grammar modals and you'll find thousands of sites, at least five of which should be of interest.

CJ