We are told that can and must are always independent verbs in the indicative, and that may, might, could, would, and should are either subjunctive auxiliaries or independent verbs parsed in the indicative, separately from the infinitives with which they seem to combine. But in parsing these words as separate verbs the student is left in doubt as to whether they are transitive or intransitive, and as to the office of the infinitives that follow.
Shall (to owe) and will (to determine) are, in their original meaning, transitive. May, can, and must denote power (hence potential); and, as the infinitive with which they combine names the act on which this power is exercised, some philologists regard them as originally transitive. Among these is our distinguished critic, Prof. Francis A. March. May denotes power from without coming from a removal of all hindrance,—hence permission or possibility. Can denotes power from within,—hence ability. Must denotes power from without coming from circumstances or the nature of things,—hence necessity or obligation. Should, would, might, and could are past forms of shall, will, may, and can.
The auxiliaries take different shades of meaning. In some constructions the meaning is fainter or less emphatic than in others. To say just how little of its common or original meaning may, can, must, shall, or will must have to be an auxiliary, and how much to be a "notional," or independent, verb would be extremely venturesome For instance, could in (6) above expresses power or ability to do, as does could in (11), yet we are told that the former could is a mere auxiliary, while the latter is an independent verb. May in (1) denotes a desired removal of all hindrance; may in (7) denotes a possible removal of hindrance. It is hard to see why the former may is necessarily a mere auxiliary, and the latter a "notional," or independent, verb. These are some of the difficulties—not to say inconsistencies—met by the student who is taught that there is no Potential Mode.
In a scholarly work revised by Skeat, Wrightson, speaking of I may, can, shall, or will love, says, "These auxiliary verbs had at some time such a clear and definite meaning that it would have been tolerably easy to determine the case function discharged by the infinitive; but these verbs, after passing through various shades of meaning, have at last become little more than conventional symbols, so that it would be worse than useless to attempt to analyze these periphrastic tenses of our moods."
A CONJUGATION OF TEACH.
Active Voice.
INDICATIVE MODE.
Present Indefinite…………He teaches.
Present Imperfect………….He is teaching.
Present Perfect……………He has taught.
Present Perfect Continuous….He has been teaching.
Past Indefinite……………He taught.
Past Imperfect…………….He was teaching.
Past Perfect………………He had taught.
Past Perfect Continuous…….He had been teaching.
Future Indefinite………….He will teach.
Future Imperfect…………..He will be teaching.
Future Perfect…………….He will have taught.
Future Perfect Continuous…..He will have been teaching.