How do you connect clauses in Korean?
Korean joins clauses with endings attached to the verb stem rather than with separate conjunctions. 고 simply lists, 아서/어서 gives cause or sequence, 니까 gives reason with more emphasis, and 는데 provides background or contrast. Choosing between them changes the relationship between the two halves, and one of them cannot precede a command.
고 lists, 서 sequences
고 joins two things with no claimed relationship between them. 아서/어서 says the first led to the second, either causally or in sequence, and the two clauses must share a subject.
밥을 먹고 잤어요.
babeul meokgo jasseoyo
I ate and slept.
Two events, no claimed link.
밥을 먹어서 배불러요.
babeul meogeoseo baebulleoyo
I ate, so I'm full.
Cause.
니까 gives a reason you can act on
Both 서 and 니까 express because, but only 니까 can be followed by a command or suggestion. This is the distinction that trips people up, because the English translation is identical.
비가 오니까 우산을 가져가세요.
biga onikka usaneul gajyeogaseyo
It's raining, so take an umbrella.
Command — needs 니까.
비가 와서 안 갔어요.
biga waseo an gasseoyo
It rained, so I didn't go.
Statement — 서 is fine.
는데 sets up background or contrast
는데 is extremely common and hard to translate, because it does several jobs: providing context before the main point, softening a statement, or marking contrast. It often ends a sentence entirely, leaving the rest implied.
한국어를 배우는데 어려워요.
hangugeoreul baeuneunde eoryeowoyo
I'm learning Korean, and it's hard.
Background then comment.
가고 싶은데…
gago sipeunde…
I'd like to go, but…
Trailing off is normal and polite.
Common mistakes
비가 와서 우산을 가져가세요.
비가 오니까 우산을 가져가세요.
아서/어서 cannot be followed by a command or suggestion. Reason plus instruction requires 니까.
밥을 먹어서 친구가 왔어요.
밥을 먹고 친구가 왔어요.
아서/어서 requires the same subject in both clauses. With different subjects, use 고.
먹어서요.
먹어서 배불러요.
어서 is a connective and needs a following clause; it cannot close a sentence the way 는데 can.
Frequently asked questions
What is the real difference between 서 and 니까?
니까 presents the reason as your own judgement, which is why it can support a command. 서 presents it as a plain fact or sequence. In practice, if a command or suggestion follows, you need 니까.
Why does 는데 appear so often?
Because it does the work several English devices do — setting context, softening, contrasting, and trailing off politely rather than stating a request outright. It is one of the most useful endings to learn early.
Can I just use 고 for everything?
You would be understood, but you would sound flat and disconnected, because 고 claims no relationship between clauses. Korean expects the connective to carry that information.