Grammar
-자
Meaning
Advancedja
Attached directly to a verb stem, -자 says the second clause followed immediately once the first happened. In experimental writing it does the work English does with 'when' or 'upon': 노출하자 크기가 감소했습니다 = 'when they were exposed, the size decreased'. It cannot take a tense marker (노출했자 is wrong), and the two clauses must have the natural order of cause before effect — which is exactly why methods sections reach for it.
Form
Attaches straight to a verb stem: stem + -자. No tense marker may come before it — 올라갔자 is wrong, 올라가자 is right, and the past tense is carried by the second clause alone. It does not attach to adjectives, and the subject of the two clauses may differ.
Examples
온도를 올리자 반응이 빨라졌습니다.
When the temperature was raised, the reaction sped up.
온도가 올라가자 반응 시간이 짧아졌습니다.
As soon as the temperature rose, the response time got shorter.
아이가 팔을 휘두르자 교사가 팔을 잡았습니다.
When the child swung their arms, the teacher took hold of them.
문을 열자 찬 바람이 들어왔습니다.
As soon as the door was opened, cold air came in.
Common mistakes & nuance
Reserved for writing. In speech Koreans say -자마자 instead, and a learner who uses -자 in conversation will sound like a news bulletin. It also refuses commands and suggestions: you cannot follow it with 하세요 or 합시다. Note the trap that -자 attached to a verb stem is also the plain suggestion ending ('let's ~'), a completely different form that happens to look identical.
Similar grammar
- -자마자
The same meaning, but -자마자 belongs to everyday speech and -자 to written or formal registers. -자마자 also stresses that no time at all passed; -자 merely puts the events in order.
- -(으)니까
-자 reports what happened next. -(으)니까 reports what the speaker found out or why they concluded something, so it carries a discovery or a reason that -자 does not.