5. Auxiliary derived from the idea of power, dependent upon circumstances external to the agent—may.
6. Auxiliary derived from the idea of power, dependent upon circumstances internal to the agent—can. May is simply permissive; can is potential. In respect to the idea of power residing in the agent being the cause which determines a contingent action, can is in the same relation to may as will is to shall.
"May et can, cum eorum præteritis imperfectis, might et could, potentiam innuunt: cum hoc tamen discrimine: may et might vel de jure vel saltem de rei possibilitate dicuntur, at can et could de viribus agentis."—Wallis, p. 107.
7. Auxiliary derived from the idea of sufferance—let.
8. Auxiliary derived from the idea of necessity—must.
"Must necessitatem innuit. Debeo, oportet, necesse est urere, I must burn. Aliquando sed rarius in præterito dicitur must (quasi ex must'd seu must't contractum). Sic, si de præterito dicatur, he must (seu must't) be burnt, oportebat uri seu necesse habuit ut ureretur."—Wallis, 107.
9. Auxiliary derived from the idea of action—do.
C. Classification of auxiliary verbs in respect to their mode
of construction.—Auxiliary verbs combine with others in three ways.
1. With participles.—a) With the present, or active, participle—I am speaking: b) With the past, or passive, participle—I am beaten, I have beaten.