려고 하다 is a grammar construct that is showing future action, and is similar to (ㄹ/을)래요 but in this case more like ‘going to do’ and is not just an ending; it acts more like a conjunction because unlike the other future forms, the ending 하다 can be treated just as any other verb. To make this ending, we conjugate the verb into present tense using 아/어/여, then we add 서 to the end. For (으)니까, we conjugate the verb with 으 if the ending is a consonant and add 니까 to the end. We can either use these as endings, or connect other sentences to them to show the logical connection.
V + (아/어/여)서
1. (아/어/여)서 is similar to using 그래서 to join sentences, except it is attached to the end of the verbs themselves, and can also be used as an ending by itself if the sentence is enough to show the reason. You can think of the sentence using (아/어/여)서 as explaining the reason behind something, and then following it up with the next sentence. (아/어/여)서 cannot be used in imperative sentences.
V + (으)니까
2. (으)니까 is similar to (아/어/여)서, except the connection is more direct and the sentence containing (으)니까 explains the reason more clearly. (으)니까 can be used in imperative sentences whereas (아/어/여)서 cannot. (으)니까 can also be shortened to (으)니 in colloquial speech.
N + (으)니까
N + (이)라서/(이)어서
3. Both can be used with nouns too. (으)니까 is just appended to the noun, but (아/어/여)서 has to become (이)라서 or (이)어서, with 이 appended if the noun ends in a consonant and either 라서 or 어서 added.
요즘에 바빠서 친구랑 못 만나요. – I was busy recently so I couldn’t meet my friends.
미국에 와서 뭐 할 거예요? – You came to USA, so what are you going to do?
친구니까요! – Because I’m your friend!
외국사람이라서 한국어 잘 못 해요. – I’m a foreigner, so I cannot speak Korean well.
저 지금 바쁘니까 나중에 전화할게요. – I’m busy right now so I’ll call you later.