昨 joins the sun 日 with 乍 (just now): the sun that was just here — yesterday. Time words in Chinese are built from suns and moons doing things.
昨 owns one word and owns it completely: 昨天 (yesterday — with 天 you learned). Notice the family: 今天 today, 明天 tomorrow (bright day coming), 昨天 the just-gone day. Chinese verbs never change tense — no 'went' or 'gone' — so these time words do all the work English grammar does. Master 昨天 and you can narrate the past with zero conjugation.
The sun 日 that was just 乍 here — YESTERDAY.
Chinese has no past tense — 昨天 (yesterday) IS the past tense. Time words replace conjugation entirely.