*Historically some of these modal auxiliaries are in the present tense and others are in the past tense.
            Present tense: can, shall, will, may
            Past tense: could, should, would, might
In modern English, the past tense modal auxiliaries can all be used to refer to the present or future, unlike other past tense forms (as in  He could be working now, I should work tomorrow, Mary might be at home now, He would come tomorrow (if we let him)).  In standard English, the present tense modal auxiliaries cannot be used to refer to the past, however, so I can work tomorrow is fine, but I can work yesterday is awful.
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