快(要) + Verb/Adjective + 了 | (Time) + 就要 + Verb + 了

快要...了 (kuàiyào...le) vs 就要...了 (jiùyào...le) — 'about to' happen soon

Both patterns sandwich the verb between a 'soon' word and a final 了 to mean something is about to happen: 电影快要开始了 (the movie is about to start), 火车就要开了 (the train is about to leave). 快(要) emphasizes that the moment is drawing near from the speaker's present vantage point, and doesn't usually combine with a specific time word — 快要明天了 sounds odd. 就要 works the same way but CAN be preceded by a specific time word to say exactly when: 他明天就要走了 (he's leaving [as soon as] tomorrow) — that combination (specific time + 就要) is not natural with 快要. The deciding question: do I have a specific time point to state before it (→就要, since it tolerates a time word in front), or am I just saying 'soon' in general, from right now (→快 or 快要, either works)? Both need the closing 了 — dropping it turns the sentence into a plain statement about the future rather than 'right on the verge'.