-What does -서 do?-
This little particle has the power to merge two sentences into one, serving as a 'because' or a 'so' equivalent. This particle illustrates a cause and effect relationship between two clauses of the sentence.
-Where does -서 stand in the sentence?-
-서 can be added to VERBS or ADJECTIVES, not nouns (!), in the 'cause' clause of the sentence. Basically, it'll all make sense in the examples :)
돈이 없어서 여행을 할 수 없어요- Because I don't have money, I won't have a trip (or I will not go on vacation)
사람들이 많아서 이 방은 시끄러워요- Because there are a lot of people, this room is noisy
요즘 바빠서 점심을 잘 먹지 못했어요- Lately I've been busy so I wasn't able to eat lunch properly
매일 한국말을 공부해서 한국말을 잘해요- Because I study Korean every day, my Korean is good
*Keep in mind: The verb or adjective with the -서 added to it does NOT indicate the tense of the sentence. That role is done by the other verb or adjective at the end of the sentence.
Above, I've mentioned instances where verbs or adjectives in the first part of the sentence affect the second part of the sentence, but there are times where nouns in the first part of that sentence have the same effect. Obviously, we can't add -서 to nouns so in this case we use verbs 'to be' and 'not to be' (이다 and 아니다)
이다 -- noun + 라서/이라서
아니다 -- noun이/가 + 아니라서
우리 친구라서 같이 공부해요- Because we are friends, we study together
저는 학생이라서 숙제가 많아요- Because I'm a student, I have a lot of homework
여기는 큰 도시가 아니라서 지하철이 없어요- Because this isn't a bid city, there is no subway
스물한 살이 아니라서 술을 못 마셔요- Because I'm not 21 I can't drink alcohol
Keep in mind: Even though in English the meaning of the sentence stays the same no matter whether the cause of the effect comes first, in Korean that will not work. The cause always has to come first and the effect follows as the second part of the sentence.
Hope this helped! Let me know if you have any questions :)