-아/어서인지

Gives a reason the speaker is not sure of: 'maybe because…, perhaps since…'. It is a compact, slightly more written equivalent of -아/어서 그런지.

이유·원인Reason & cause

Han: An uncertain cause cannot back up a request; use -(으)니까 for a reason behind a command.

Examples

시험이 끝나서인지 학생들 표정이 한결 밝아 보였다.Maybe because the exams were over, the students looked much more cheerful.

어젯밤에 잠을 못 자서인지 하루 종일 머리가 아파요.Maybe because I couldn't sleep last night, I've had a headache all day.

주말이어서인지 시내에 사람이 평소보다 훨씬 많았다.Perhaps because it was the weekend, the city centre was much busier than usual.

-아/어서인지vs-아/어서-아/어서 states the cause as certain; 서인지 marks it as a guess: 늦게 자서 피곤해요 vs 늦게 자서인지 피곤해요.

V/A+아서인지/어서인지 (하다 → 해서인지)

N+이어서인지 (after a consonant)

여서인지 (after a vowel), or N+(이)라서인지

피곤하다 → 피곤해서인지; 춥다 → 추워서인지; 듣다 → 들어서인지; 주말 → 주말이어서인지/주말이라서인지

The first clause is your guess at the cause; the second is a fact you observed. Because the cause is uncertain, a command or suggestion cannot follow.
Irregular stems change as with -아/어서: 추워서인지, 들어서인지, 몰라서인지.
It belongs to a family of 'maybe because' forms built with 인지: -기 때문인지, -(으)ㄴ 탓인지, N 때문인지, N 덕분인지.

Wrong: ~~날씨가 추워서인지 따뜻하게 입으세요.~~

Right: 날씨가 추우니까 따뜻하게 입으세요.

An uncertain cause cannot back up a request; use -(으)니까 for a reason behind a command.

Hannim plans each day up to your exam date: questions on this pattern, reviews, full mock papers and graded writing. 7 days free, no card.