Very similar to -(으)려면, but less common. Expresses that if the listener intends to take an action, then they should act according to the second clause (the part that comes after the (으)려거든)
Examples
건강한 치아를 유지하<span class='focus'>려거든</span> 하루에 두번 이 닦아야 해요.
If you want to maintain your dental health, you have to brush your teeth twice a day.
여러 나라의 요리를 배우<span class='focus'>려거든</span> 요리학원에 등록해보세요.
If you want to learn how to cook food from a lot of different countries, try enrolling in a cooking hakwon.