Myth check
了 is the Mandarin past tense.
了 often marks a change or a finished event; English “-ed” is the wrong map.
Verdict
mostly_false
Why
我饿了 is about now. 下雨了 is rain starting. Neither needs a past-tense story.
Some finished events take 了 (我吃了). Many time-marked sentences do not stamp 了 on every verb.
Negating a completed event usually wants 没, not a 不了 sandwich you invented from English.
Better framing
Ask what changed or what finished. If nothing did, you may not need 了.
Practice tip
Say 我饿了, 太贵了, and 我没吃 as a set — two 了, one 没 — then open the 了 pattern page.