-기 마련이다

-기 마련이다 says that something naturally happens as a general rule of life, so it is no surprise.

추측·판단Guessing & judging

Han: -기 마련이다 states general truths, so it cannot predict one specific event like tomorrow's weather; use -(으)ㄹ 거예요 for that.

Examples

시간이 지나면 기억은 흐려지기 마련이에요.As time passes, memories naturally fade.

꾸준히 노력하면 좋은 결과가 있기 마련입니다.If you keep working steadily, good results are bound to come.

외국어는 매일 쓰지 않으면 잊어버리기 마련이에요.If you don't use a foreign language every day, you naturally forget it.

처음에는 누구나 실수하기 마련이에요.Everyone is bound to make mistakes at first.

V/A+기 마련이다

실수하다 → 실수하기 마련이다, 없다 → 없기 마련이다

Use it for general truths and natural outcomes, not for predictions about one specific event (✗ 내일은 비가 오기 마련이에요).
Do not put a past-tense marker before -기 (✗ 했기 마련이다); the verb stays in its plain form.
It is a statement, so it cannot end a command or suggestion; it often follows a condition like -(으)면 or words like 누구나.
-게 마련이다 has the same meaning and can be used interchangeably.

Wrong: ~~내일은 비가 오기 마련이에요.~~

Right: 내일은 비가 올 거예요.

-기 마련이다 states general truths, so it cannot predict one specific event like tomorrow's weather; use -(으)ㄹ 거예요 for that.

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