Bound to, natural to: 게/기 마련이다

V + 게/기 마련 이다 can be used to show that something is bound to happen, that it is natural or even fate. 마련 literally means ‘arrangement’ or ‘prepare’, so the structure suggests that you should expect the outcome, or that it is common knowledge:

열심히 공부하면 더 똑똑해지게 마련 이에요.

If you continue to study hard, you’re bound to get smarter.

여자친구가 오래 멀리 떨어질 때져 있을 때 우울하게 마련 이에요.

When your girlfriend is far away for so long it’s only natural to feel depressed.

제가 남편에게 계속 과식을 하면 허리에 살이 찌기 마련 이라고 말했어요.

I told my husband that if he continues to overeat, he’s bound to put on weight around the waist.

어린이들은 부모에게 반항하게 마련 이에요.

It’s only natural that children rebel against their parents.

한국어를 배우면서 실수하기 마련 이지만 괜찮아요.

It’s natural to make mistakes when learning Korean but that’s OK.

추석의 마지막 날에는 교통이 복잡하게 마련 이에요.

It’s expected that there is traffic congestion on the last day of Chuseok.