The form -(으)니까 is one you use for a couple of things. The first use is to express the reason or cause of something. The other use is expressing discovering something as a result of a performed action. It is a rather simple form, yet very important as it is used to form other more advanced Korean grammar forms like -(으)ㄹ 테니까. Lastly, the connective adverb 그러니까 is derived from this form.
Reason And Cause
As mentioned earlier, the first and main use of -(으)니까 is to express the reason or cause of something. In this regard, it is very similar to the basic form -아/어서. However, there are some slight, yet very important differences. As a result, you will use one rather than the other depending on the situation.
-(으)니까 expresses a subjective reason or offers the basis for what it said. Furthermore, the listener also knows about the topic that is being discussed. As such, this form allows the use of imperative forms, like -(으)세요 and -아/어 주세요, and propositive forms, like -(으)ㅂ시다 and -(으)ㄹ까요. Keep these facts in mind and you won’t make any mix-ups when selecting between -(으)니까 and -아/어서.
The connective adverb 그러니까 is derived from this form and thus has the same guidelines.
The form -(으)니까 is a lot more difficult to use than -아/어서. This is mainly due to the fact, that depending on the situation it can actually be rude to express yourself with it to others. This is mainly true when offering excuses to others. If you use -(으)니까 when offering excuses, it can come across as an accusation when not properly formulated. When not formulated properly, it sounds as if you tell the other person, he should’ve been aware of the situation.
가: 오늘 어떻게 출근할까요?
나: 길이 막히니까 지하철로 가요.
A: How are we going to work today?
B: Traffic is jammed so let’s go with the subway.
The other use of the form -(으)니까 is to express your discovery of something upon performing a certain action. The first clause of the sentence is the action that leads to the discovery, while the second clause expresses what was discovered. It is rather simple to use and understand.
가: 드디어 크리스마스 선물이 왔나요?
나: 저는 정말 놀랐어요. 아침에 일어나니까 선물이 많았어요.
A: Did your Christmas presents finally arrive?
B: I was really surprised. When I woke up in the morning there were many presents.
How To Use
The form -(으)니까 is a bit more complicated to use than -아/어서. However, its conjugation is rather simple. As you might be able to tell, the form uses the ‘batchim/no batchim’ rule. Thus you need to check for whether there is a batchim or not. If there is a batchim, you use -으니까, if not, you use -니까. Keep the irregular verbs in mind though.
There are also a few things you need to keep in mind. Tense markers, such as -겠- and -았/었-, can be used with -(으)니까. However, you cannot use them when you use it to express discovery. Furthermore, you cannot use it with common words used for greetings and such. These idioms use the form -아/어서. Lastly, when you express discovery with this form you can only attach it to verbs.