~기/게 마련이다 (~gi/ge ma-ryeo-ni-da) – it is bound to be…/it is natural for it to be…/it is surely to be…
There are many ways of translating this grammar particle, depending on the context of the sentence.
The main gist of this phrase is to express that a particular situation or happening is certain or justified.
You can think of this grammar particle as expressing “it is just a matter of fact”.
This is not to be confused with 마련하다, which means to prepare or arrange.
어린들이 놀이터에서 놀면 다치기 마련이야, 그대로 다음에 놀 때 조심히 하지.
When kids play at the playground, it is natural for them to get hurt, that way they will be more careful when they play next time.
시간은 잘 활용하든지도 흐르기 마련이라서 자기 위해 의미 있는 일을 해야지.
No matter whether you utilise your time or not, time is bound to pass so you should do meaningful things for yourself.