Contrast the impersonal verb "rain" (all the forms of which exist, even if they sometimes look semantically odd) with the defective verb "can" (only "I can" and "I could" are possible). In most cases, a synonym for the defective verb must be used instead (i.e. "to be able to"). (The forms with an asterisk (*) are impossible, at least with respect to the relevant sense of the verb; these phonemes may by coincidence be attested with respect to a homograph with "canning" = "the act of preserving and packaging in cans".)