In the first of these examples, the auxiliary may implies possibility; in the second it implies liberty; that is, he is at liberty to go or to stay; in the third, must denotes necessity; can denotes power or ability; would implies will or inclination; that is, he had a mind to walk; and should implies obligation.  Hence you perceive, that the verbs, may rain, may go, must eat, must drink, can ride, world walk, and should learn, are in the potential mood.

NOTE 1.  As a verb in the indicative mood is converted into the subjunctive when it is preceded by a conjunction expressing doubt, contingency, supposition, &c., so a verb in the potential mood, may, in like manner, be turned into the subjunctive; as, “If I could deceive him, I should abhor it; Though he should increase in wealth, he would not be charitable.”  I could deceive, is in the potential; If I could deceive, is in the subjunctive mood.

    2.  The potential mood, as well as the indicative, is used in asking
    a question; as, “May I go?  Could you understand him?  Must we die?”

The INFINITIVE MOOD expresses action, passion, or being, in a general and unlimited manner, having no nominative, consequently, neither person nor number; as, "To speak, to walk.”

Infinitive means unconfined, or unlimited.  This mood is called the infinitive, because its verb is not confined or limited to a nominative.  A verb in any other mood is limited; that is, it must agree in number and person with its nominative; but a verb in this mood has no nominative, therefore, it never changes its termination, except to form the perfect tense.  Now you understand why all verbs are called finite or limited, excepting those in the infinitive mood.

NOTE. To, the sign of the infinitive mood, is often understood before the verb; as, “Let me proceed;” that is, Let me to proceed.  See RULE 25. To is not a preposition when joined to a verb in this mood; thus, to ride, to rule; but it should be parsed with the verb, and as a part of it.

If you study this lecture attentively, you will perceive, that when I say, I write, the verb is in the indicative mood; but when I say, if I write, or, unless I write, &c. the verb is in the subjunctive mood; write thou, or write ye or you, the imperative; I may write, I must write, I could write, &c. the potential; and to write, the infinitive.  Any other verb (except the defective) may be employed in the same manner.

* * * * *

II.  OF THE TENSES.

TENSE means time.

Verbs have six tenses, the Present, the Imperfect, the Perfect, the
Pluperfect, and the First and Second Future tenses.