Can is used to denote the ability of the subject.
May is used to denote permission, possibility, purpose, or desire. Thus the request for permission should be, "May I?" not "Can I?"
Must indicates necessity.
Ought expresses obligation.
Had should never be used with ought. To express a moral obligation in past time, combine ought with the perfect infinitive: [I ought to have done it].
Should sometimes expresses duty: [You should not go].
Would sometimes denotes a custom: [He would sit there for hours]. Sometimes it expresses a wish: [Would he were here!]. For other uses of should and would, see Appendix 60.
+55. Principal Parts.+—The main forms of the verb—so important as to be called the principal parts because the other parts are formed from them— are the root infinitive, the preterite (past) indicative, and the past participle [move, moved, moved; sing, sang, sung; be, was, been]. The present participle is sometimes given with the principal parts.
+56. Inflection.+—As is evident from the preceding paragraph, verbs have certain changes of form to indicate change of meaning. Such a change or inflection, in the case of the noun, is called declension; in the case of the verb it is called conjugation. Nouns are declined; verbs are conjugated.
+57. Person and Number.+—In Latin, or any other highly inflected language, there are many terminations to indicate differences in person and number, but in English there is but one in common use, s in the third person singular: [He runs], St or est is used after thou in the second person singular: [Thou lovest].