Once you've got your head round the fact that you say everything backwards, you won't be surprised to hear that there's another ending which also means "because" or "so" in English: -(으)니까(-eu)ni-kka. This follows these rules, but basically if there's a consonant (받침bad-chim / a written final consonant) at the end you write -으니까-eu-ni-kka and if there isn't you add -니까-ni-kka.
-(으)니까(-eu)ni-kka goes in exactly the same place as -아서/어서-a-seo/eo-seo and also the order is backwards in exactly the same way. But the meaning is very different and this difference does not exist in English.
The difference
This is long, but bear with me, and I hope it helps you understand.I was waiting for someone in the freezing cold, and I thought to myself, "how do I know when they've arrived?". The bus stop was further up the road from me, so I knew if a bus went by it could mean my friend had arrived. But I didn't know which bus and what time exactly. So when two buses went by, I thought to myself "My friend has arrived because at least two buses have gone by (she must have been on one). But she didn't arrive on either one! So the two buses actually did not mean she had arrived, they only made me think she had arrived. She could be on any bus!
And then I realised: in Korean we would say 버스 두대 지나 갔으니까 친구 왔어요beo-seu du de ji-na gass-eu-ni-kka chin-gu wass-eo-yo / because two buses went past my friend has arrived because how the hell can two buses directly mean that someone has arrived! only one bus can mean that! (unless you can travel on two buses at once)
-아서/어서-a-seo/eo-seo is used for things which are caused directly. It rains so you get wet, you're slow because you're fat etc. etc. -(으)니까(-eu)ni-kka is used when you think things are caused, or do things because of something. The two buses going by made me think my friend had arrived. It didn't cause her to arrive, because she wasn't on them.
This is a bit difficult to get your head around, but have a look at these sentences in English and think about it. Does the situation make you do something (-(으)니까(-eu)ni-kka), or does the situation cause something to happen(-아서/어서-a-seo/eo-seo):
- Because It's raining, I brought my umbrella-(으)니까 because rain doesnt cause you to majically have an umrbella!
- I ate a lot because I was hungry-아서/어서 because hunger causes you to eat, there is a direct relationship
- He drove badly because he was really drunk-아서/어서 because drinking does make you drive badly - they are directly related
- The lady drove carefully because it was snowing-(으)니까 because snow doesn't automatically make everyone a good driver. but if you're talking about someone who IS a good driver, then you can use -아서/어서
- It's dangerous because there's lots of snow--아서/어서 because snow DOES make driving dangerous
- She's beautiful because she has big eyesBOTH!!!! hahaha
I included the last one to show that both are possible in some cases. If her big eyes are the cause of her beauty, then we use -아서/어서-a-seo/eo-seo but if her big eyes make you think she's beautiful (but she isn't according to everyone) then we can use -(으)니까(-eu)ni-kka.
As with anything complicated like this, to learn the difference and use these things properly, it just takes time. I've been speaking Korean for a year now, and it only hit me now, exactly what the difference was. What you have to do is constantly practise and think about what you're saying and doing.
If you're still confused, read on, you can get in touch and I will give more details, examples and explanations
