Giving Reasons: Because/Since ((으)니까)

This is the most crucial distinction. The main differences are:

• Commands/Suggestions: -(으)니까 can precede commands (-(으)세요), suggestions (-자), and propositions (-(으)ㄹ까요?). -아/어서 cannot.
• Past Tense: -(으)니까 can attach to past tense stems (-았/었/였으니까). -아/어서 cannot be directly preceded by a past tense marker (you don't say 먹었어서).
• Subjectivity vs. Objectivity: -(으)니까 often implies a personal judgment, immediate observation, or newly discovered fact by the speaker. -아/어서 tends to describe a more objective, natural, or inherent cause-and-effect relationship. It is used for universal truths, thanks, and apologies.
• Sequencing: -아/어서 can imply a sequence of events where the first action directly leads to the second, without necessarily being a 'reason' for it (e.g., 밥을 먹어서 배가 불러요. – I ate, so I'm full vs. 밥을 먹었으니까 배가 불러요. – Since I ate, I'm full). -(으)니까 is more pointedly about the reason for the consequence or action.

Yes, it can. When used with formal sentence endings like -ㅂ니다/습니다 or -(으)십시오, it is perfectly formal. However, because it expresses a direct reason often leading to a command or strong suggestion, it can sound more direct than other reason-giving expressions. When addressing someone much higher in status, -아/어서 or -기 때문에 might sometimes be preferred for a softer approach, especially if the reason is not a direct instruction.

-(으)니깐 is a more emphatic and often more casual variation of -(으)니까. The addition of ㄴ (-으니까 → 으)니깐 or 니까 → 니깐) adds a slight nuance of insistence or stronger emphasis to the reason. You will frequently hear -(으)니깐 in spoken, casual Korean, especially when someone wants to make their reason very clear or persuasive, like 내가 먼저 왔으니까 내가 먹을게! (I came first, so I'll eat it!). It's less common in formal writing.

Yes, you can express future reasons using the -(으)ㄹ 거니까 structure. This indicates that a future event or intention serves as the reason for the current or subsequent action. For example, 내일 회의가 있으니까 오늘 미리 준비해야 해요. (Since there's a meeting tomorrow, I need to prepare in advance today.) Here, the future meeting is the reason for the present preparation.

-기 때문에 (because of/due to) is generally more objective and formal than -(으)니까. It places stronger emphasis on the cause itself, similar to 'the reason is...' or 'due to the fact that...'. While -(으)니까 often leads to commands/suggestions and expresses subjective reasons or discoveries, -기 때문에 typically states a more neutral, undeniable cause-and-effect, and does not generally precede commands or suggestions. It is also commonly used in written contexts or for formal reports. For instance, 교통 체증 때문에 늦었어요. (I was late because of traffic congestion.) is a factual statement of cause.