Could

Could \Could\ (k??d), imp. of {Can}. [OF. coude. The l was
   inserted by mistake, under the influence of should and
   would.]
   Was, should be, or would be, able, capable, or susceptible.
   Used as an auxiliary, in the past tense or in the conditional
   present.
   [1913 Webster]
    

Can \Can\, v. t. & i.

   Note: [The transitive use is obsolete.] [imp. {Could}.] [OE.
         cunnen, cannen (1st sing. pres. I can), to know, know
         how, be able, AS. cunnan, 1st sing. pres. ic cann or
         can, pl. cunnon, 1st sing. imp. c[=u][eth]e (for
         cun[eth]e); p. p. c[=u][eth] (for cun[eth]); akin to
         OS. Kunnan, D. Kunnen, OHG. chunnan, G. k["o]nnen,
         Icel. kunna, Goth. Kunnan, and E. ken to know. The
         present tense I can (AS. ic cann) was originally a
         preterit, meaning I have known or Learned, and hence I
         know, know how. [root]45. See {Ken}, {Know}; cf. {Con},
         {Cunning}, {Uncouth}.]
   1. To know; to understand. [Obs.]
      [1913 Webster]

            I can rimes of Robin Hood.            --Piers
                                                  Plowman.
      [1913 Webster]

            I can no Latin, quod she.             --Piers
                                                  Plowman.
      [1913 Webster]

            Let the priest in surplice white,
            That defunctive music can.            --Shak.
      [1913 Webster]

   2. To be able to do; to have power or influence. [Obs.]
      [1913 Webster]

            The will of Him who all things can.   --Milton.
      [1913 Webster]

            For what, alas, can these my single arms? --Shak.
      [1913 Webster]

            M[ae]c[ae]nas and Agrippa, who can most with
            C[ae]sar.                             --Beau. & Fl.
      [1913 Webster]

   3. To be able; -- followed by an infinitive without to; as, I
      can go, but do not wish to.

   Syn: {Can but}, {Can not but}. It is an error to use the
        former of these phrases where the sens requires the
        latter. If we say, "I can but perish if I go," "But"
        means only, and denotes that this is all or the worst
        that can happen. When the apostle Peter said. "We can
        not but speak of the things which we have seen and
        heard." he referred to a moral constraint or necessety
        which rested upon him and his associates; and the
        meaning was, We cannot help speaking, We cannot refrain
        from speaking. This idea of a moral necessity or
        constraint is of frequent occurrence, and is also
        expressed in the phrase, "I can not help it." Thus we
        say. "I can not but hope," "I can not but believe," "I
        can not but think," "I can not but remark," etc., in
        cases in which it would be an error to use the phrase
        can but.
        [1913 Webster]

              Yet he could not but acknowledge to himself that
              there was something calculated to impress awe, . .
              . in the sudden appearances and vanishings . . .
              of the masque                       --De Quincey.
        [1913 Webster]

              Tom felt that this was a rebuff for him, and could
              not but understand it as a left-handed hit at his
              employer.                           --Dickens.
        [1913 Webster]