169. The tenses of the Perfect System in the active voice are made from the Perfect stem as follows.
a. In the Perfect Indicative the endings -ī, -istī, etc. are added directly to the Perfect stem.
b. In the Pluperfect Indicative the suffix -eram, -erās, etc. is added to the Perfect stem.
Note— This seems to represent an older †-is-ām, etc. formed on the analogy of the Future Perfect in -erō (older †-is-ō, see c below) and influenced by eram (imperfect of sum) in comparison with erō (future of sum).
c. In the Future Perfect the suffix -erō, -eris, etc. is added to the Perfect stem.
Note— This formation was originally a subjunctive of the -s- Aorist, ending probably in †-is-ō. The -is- is doubtless the same as that seen in the 2nd person singular of the Perfect Indicative (vīd-is-tī), in the Perfect Infinitive (vīd-is-se), and in the Pluperfect Subjunctive (vīd-is-sem), s being the aorist sign and i probably an old stem vowel.
d. In the Perfect Subjunctive the suffix -erim, -eris, etc. is added to the Perfect stem.
Note— This formation was originally an Optative of the -s- Aorist (-er- for older
-is-, as in the Future Perfect, see c above). The i after r is the Optative mood-sign ī shortened (see § 168.e, Note 2). Forms in -īs, -īt, -īmus, -ītis, are sometimes found. The shortening in -ĭs, -ĭmus, -ĭtis, is due to confusion with the Future Perfect.
e. In the Pluperfect Subjunctive the suffix -issem, -issēs, etc. is added to the Perfect stem.
Note— Apparently this tense was formed on the analogy of the Pluperfect Indicative in †-is-ām (later -er-am, see b.), and influenced by essem (earlier †essēm) in its relation to eram (earlier †esām).1