'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.