1. yes. 걸 is for 것을 'verb + ㄴ것' makes something(verb or adjective) to noun so 'telling my secret' is '비밀을 말한 것' regret='후회하다' i regret telling my secret 나는 나의 비밀을 말한 것을 후회한다 2. ㄹ걸 is not future tense. it's means "past tense" 'verb or adjective + ㄹ걸' means "I should have verb but i didn't" similarly ,'verb or adjective + 않을(말) 걸' means "I should have verb but i did" 3. 괜히 means "shouldn't have" in many sentences. usually '괜히 한번' means pointless, for no reason. if "괜히" means pointless, for no reason. verb should be present or future tense. i think. 괜히 something 먹었다. ( i shouldn't have eaten something) 괜히 한번 something을 차봤다 ( i kicked something for no reason) 괜히 something을 차본다 ( i kick something for no reason)