힘들다 1) 공부가 힘들다 : Study is hard. 2) 인생이 힘들다 : Life is difficult. 3) 요리는 힘들다 : Cooking is hard. 어렵다. 1) 수학은 어렵다 : Math is hard. 2) 스마트폰 배우기는 어렵다 : Learning smart phone is hard. 3) 이해하기 어렵다 : Hard to understand. ㅋㅋ these two word is kinda same. both of them are used in same sentence. but, 힘들다 means physical difficulty, and 어렵다 means .. emotional difficulty .. ^^ .

As I understand it: 힘들다 - tough, tired, challenging (mentally or physically) eg. 사랑하기 너무 힘들다. 어렵다 - hard, difficult (something hard to do) eg. 숙제는 어렵다.

think of 힘들다 as using up your strength and 어렵다 as something that's hard/difficult but doesn't necessarily have to use up your strength