-기 일쑤다

-기 일쑤다 says that something undesirable happens again and again, almost as a bad habit.

추측·판단Guessing & judging

Han: The past tense is marked only on 일쑤다 (일쑤였어요); the verb before -기 stays in its plain stem.

Examples

일이 바쁠 때는 점심을 거르기 일쑤입니다.When work is busy, I often end up skipping lunch.

저는 아침잠이 많아서 회사에 지각하기 일쑤예요.I'm not a morning person, so I'm often late for work.

어렸을 때 동생은 우산을 잃어버리기 일쑤였어요.As a child, my younger brother was always losing his umbrella.

겨울에는 길이 미끄러워서 넘어지기 일쑤다.In winter the roads are slippery, so people often fall.

V+기 일쑤다

늦다 → 늦기 일쑤다 (늦기 일쑤예요 / 늦기 일쑤였어요)

It is used for negative or unwanted repeated events. With positive actions (getting praised, helping) it sounds odd; use -곤 하다 instead.
Tense goes on 일쑤다, not on the verb: 지각하기 일쑤였어요, not 지각했기 일쑤였어요.
It describes a repeated tendency, so it does not fit a single event with a specific time like 어제 or 그날.
It is not used in commands or suggestions (-(으)세요, -(으)ㅂ시다).

Wrong: ~~학생 때는 수업에 지각했기 일쑤였어요.~~

Right: 학생 때는 수업에 지각하기 일쑤였어요.

The past tense is marked only on 일쑤다 (일쑤였어요); the verb before -기 stays in its plain stem.

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