I feel like there is so much confusion on how this is taught. I kind of vaguely understood what the difference was for years and years until only recently when I came across an example that cleared it up for me.
First of all I’m just going to lay out some basic definitions of what each grammar point means
-자마자: this only ever means one thing. As soon as. Period.
-자 has two uses. The first is saying something has two characteristics. Link here to sample sentences if you’re interested
The second use is the one that is confusing. It’s confusing because it’s taught as “it’s similar to 자마자 but for more formal use” or “it’s like 자마자 but the second action doesn’t follow as quickly after the first one”. I can’t say that they’re wrong but they’re not good explanations.
I don’t have a definition, but I’ll show you through these sample sentences.
- 창문을 열자 바람이 들어왔다: the wind came in as I opened the door
- 의자에 앉자 잠들었다: I fell asleep as I sat down
- 차에서 연기가 나자 운전사는 차에서 뛰쳐나왔다. The driver rush out of the car as smoke started coming out of it
Boom. No definition needed. You can just feel what it means. Its like trying to say that I fell asleep as I sat down is similar to I fell asleep as soon as I sat down. They are somewhat similar, but really not the same at all.
One thing to note with 자 is that it isn’t used very common in spoken Korean. If you want a replacement, using 여니까 or 앉으니까 is a good choice. If you’re not sure what 니까 means here, see this post here.