該grammar point · tier 1 · 該 — bare modal: it's time to / it falls to one to (should, ought to, often 該…了)
· gāi
Auxiliary before a verb, the spoken short form of 應該: the situation has come due and the action now falls to the subject — it's my turn / time to do it. Often closed with 了.
Same word, two weights. 該 is the spoken short form and leans on a now-come-due reading: the moment has arrived and the act falls to you (我該走了 — time for me to go). 應該 is the fuller modal weighing what is the right thing to do, with no timing pressure (作業應該準時交 — homework ought to be handed in on time). 該 says it's time; 應該 says it's proper.
要…了
該…了 says the act now falls to the subject as the fitting thing to do — it's their turn (我該回家了, I should head home). 要…了 forecasts an event about to land on its own (火車要開了, the train is about to leave). 該 lays a duty come due on the subject; 要…了 reads the imminence of an event.
In 該…了 the 了 marks the moment turning — now is the time the act falls due. Bare 了2 alone marks a change of state with no duty attached (下雨了, it's raining now). 該 supplies the 'falls to you to act,' which 了 alone never carries.
negating with 沒: 我沒該走 → 我不該走 (I ought not to go)
noun after 該: 該家了 → 該回家了（該 + verb for the act）
reading 該…了 as a completed action: 我該回家了 means it's time to go home now, not that the going-home already happened
English 'should' is one flat word, so learners default to the fuller 應該 everywhere and miss the lighter spoken 該…了 that a native reaches for when the moment has just come due; the timing 了 carries the 'now it's time' that English leaves to context.