[875.] The perfect potuī to the present possum ([751]) is from a lost present *poteō, *potēre ([922]). pōnere (for *po-sinere, [112]; [170, 2]) forms an old perfect posīvī ([964]), later posuī, as if pos- were the stem.
[876.] The perfect subjunctive stem ends in -erī-, for which -eri- is sometimes used ([35, 2, 3]): as,
rēxeri-m, rēxerī-s, rēxeri-t, rēxerī-mus, rēxerī-tis, rēxeri-nt.
[877.] In the perfect subjunctive, long ī is found before the person endings -s, -mus, and -tis, some 25 times, as follows: -īs, 18 times (Plaut. 3, Pac., Enn., Ter., Hor., Tib., Sen., inscr., once each, Ov. 8), -īmus, 4 times (Plaut. 3, Ter. 1), -ītis, 3 times (Plaut. 2, Enn. 1).
[878.] In the perfect subjunctive, short i is found, as in the future perfect, some 9 times, thus: -is, 8 times (Plaut. in anapests 3, Verg. 2, Hor. 3), -imus once (Verg.). But before -tis, short i is not found.
[879.] One verb only, meminī, remember, has a perfect imperative; in this imperative, the person endings are not preceded by a vowel, thus: memen-tō, memen-tōte.
[880.] The pluperfect indicative stem ends in -erā-, which becomes -era- in some of the persons: as,