Pluperfect - Wikipedia
https://en.wikipedia.org/wiki/PluperfectThe pluperfect (shortening of plusquamperfect ), usually called past perfect in English, is a type of verb form, generally treated as a grammatical tense in certain languages, relating to an action that occurred prior to an aforementioned time in the past. Examples in English are: "we had arrived"; "they had written".
PLUPERFECT SUBJUNCTIVE - GRAMMAR DANCE
www.grammardance.com/p/pluperfect-subjunctive.htmlPluperfect Active Subjunctive Song (to the tune of Go Tell it on the Mountain) Pluperfect active subjunctive! -isse and the endings on the perfect stem:-issem, -issēs, -isset, -issēmus, -issētis, -issent. Amāvissem, amāvissēs, amāvisset, amāvissēmus, amāvissētis, amāvissent.