ĵus

Ĵus points backwards and only backwards: it marks something finished a moment ago, so it lives with the past tense and never with -os. That is the tidy half. The trap is tuj, which learners reach for because English "just" covers both: tuj means immediately and leans forward, mi tuj venos. Keep them apart and you get the English present perfect for free, since mi ĵus finis is exactly "I have just finished". Like hodiaŭ, ankoraŭ and preskaŭ, it is a primitive adverb ending in -aŭ-shaped stubbornness, so it never takes -e and never inflects.