Rather than learning the rules of when to use what, you'll have to remember the established words and expressions using them. 하루: "one day" (the 24 hour period) - 하루의 일과: The daily tasks. - 하루가 길어졌다: The day [daylight] got longer. - 하루 벌어 하루 산다: Earn a day to live a day => Live from hand to mouth. 날: a common word and a suffix for "day". Makes many words with "day" in it. - 이날(this day), 그날(that day), 전날(the previous day), 다음날(the next day) - 노는 날(holiday; off day), 기쁜 날(joyful day), 슬픈 날(sad day), 시험날(exam day) - 갠 날(clear day), 흐린 날(overcast day), 비오는 날(rainy day), 눈오는 날(snowy day) 일: a hanja(Chinese character)-based word for "day". Used for naming, counting, etc. - 7월 20일(July 20th), 15일 동안(for 15 days), (온)종일(all day long), 평일(weekday) - 일요일(Sunday), 월요일(Monday), 화-, 수-, 목-, 금요일(Friday), 토요일(Saturday) - 근무일(workday), 휴일(off day), 공휴일(holiday), 생일(birthday), 내일(tomorrow) - 결혼기념일 (wedding anniversary day), 합격자 발표일(screening result announcement day)

일 means “day” but it is never used alone (if it is used alone, it means ‘work’). 하루 which means “one day.” 하루 is used much more than 일 일 (1일). 저는 하루 동안 여행했어요 = I traveled for 1 day 날 is another word that means “day” and can be used as a stand alone word, but not counted. It is used when talking about a specific day by itself. For example: 저는 그 날에 갔어요 = I went on that day 저는 그 날에 계획이 있어요 = I have plans on that day //I got this from howtostudykorean.com, which is a very helpful website. I really recommend you use it!//