-길래 is a colloquial form of -기에 = -기 때문에 it is connective ending denoting a cause of action. e.g. 무슨 일이길래 이렇게 시끄러워? 배가 고프길래 라면을 끓여 먹었다. -는데 used for describing beforehand about a situation which is related with the next clause in order to explain, ask, suggest something. It can be used for attracting attention or giving suspense. e.g. 내가 텔레비전을 보고 있는데 전화벨이 울렸다. 그 애는 노래는 잘 부르는데 춤은 잘 못 춰. 눈이 오는데 차를 몰고 나가도 될까? The difference between '으니까' and '아/어서' 1) is used for the reason of subject. And '아/어서' is used for the general reason. Thus '으니까' can be used with '으세요'(order) or '읍시다'(suggestion). But '-아/어서' cannot be used with order or suggestion. e.g. 날씨가 더우니까 창문을 열었어요(O) (The weather is hot for the speaker) 날씨가 더워서 창문을 열었어요(O) (The weather is hot for everybody) 날씨가 더우니까 창문을 엽시다. 날씨가 더우니까 창문을 여세요. (O) 날씨가 더워서 창문을 엽시다. 날씨가 더워서 창문을 여세요. (X) 2) '으니까' can be used with past tense but '아/어서' cannot be used with past tense. e.g. 이 일은 내가 했으니까 너는 다른 일을 해라.(O) 이 일은 내가 했어서 너는 다른 일을 해라.(X) - 그책을 읽어보니 좋아요. (The book makes the speaker feed good) - 그책을 읽어서 좋아요. (The book makes feel good for everybody who read that book) - 그 사람이 서울에 가니 불러보세요. - 그 사람이 서울에 가서 불러보세요.(X) (order cannot be used with '아/어서') - 바다를 보니 기분이 좋군요. (I feel good after seeing the sea) - 바다를 봐서 기분이 좋군요. (You/He/She/They feel good after seeing the sea)

'는데' is preceded by a verb and is used when you answer the question if someone ask you where it is or what you are doing. For example, I'm doing homework = 숙제하고 있는데, your pen is one the table = 네 펜 거기 있는데. Or When you want to say something opposed to other people's opinion. e.g) If someone said to you the machine broke, but it turned out to be in a good condition, you could say "잘 되는데' / 길래 is used when you give a reason for what you've done because your heard from someone and it is preceded by a verb as well. e.g) If someone asked you why you brought an umbrella and you wanted to say that you heard from a forecast, you could say, "기상청에서 비온다길래". / 니까 is preceded by a noun! And it's used when you give a reason or an opinion that something is expected in general. For example. As a man, you should be stronger than woman(sorry for a poor example, running out of my concentration) = 남자니까 여자보다 강해야되. Lastly, '서' is used in various contexts. It can be either a preposition meaning 'at' or phrase 'so that'. e.g) as for 'so that', I'm so tired that I can't do anything. = 너무 피곤해서 아무것도 할수 없어. I hope you find what I wrote useful.