으니 grammar means Because~, therefore/so (“since; because) in the Korean language. (으)니 is a formal and short form of -(으)니까. -(으)니 is similar to 때문에 in Korean.

으니 grammar

으니 grammar is used on cause-effect type ending. (으)니 is used after a verb or adjective ending.
In formal sentences, 까 is often omitted and just say -(으)니. It is often used in an interrogative sentence or ‘let’s’ sentence.

If there is no final consonant (받침) 받침 or if the adjective/verb stem has ㄹ as a 받침 A/V + 니 is used. 가다 → 가니.
verb/adjective(consonant) + 으니까

If there is a final consonant (받침) at the end of the adjective/verb stem A/V + 으니 is used. 있다 → 있으니
verb/adjective(vowel) + 니까.

으니 grammar example

내가 실수 했으니 내가 사과할게. It’s my mistake, so I’ll apologize.
버스는 복잡하니 열차 타자. Let’s take a train because the bus is crowded.
시간이 없으니 빨리 먹자. Let’s eat quickly because we don’t have enough time.
차가 이렇게 밀리니 어떻게 운전 하지요? Since we are in a traffic jam, how should I drive?
오늘은 날씨가 추우니 따뜻한 옷을 입으세요. Since it’s cold today, wear some warm clothes.
선생남이 아직도 안 들어오니 웬일일까요? Since the teacher is not home yet, what do you think happened?

오늘은 날씨가 추우니 재킷을 입으세요. Since it’s cold today, wear a jacket. 추위 is an adjective and (으)니 is a connector which is used to show cause and effect of cold weather.

시간이 없으니까 빨리 먹자 – Since we don’t have much time, let’s eat quickly.
내일 시간이 없으니까 빨리 하자. – Since we don’t have much time tomorrow, let’s do it quickly.
더우니까 선풍기 켤까요? = It’s hot, so shall we turn on the fan?
냉장고에 밥 있으니까 먹어. = There is some rice in the refrigerator so eat it
여기 좀 시끄러우니까 다른 방에 가는 게 어때요? It is loud here, how about going to another room?
술을 많이 마셨으니까 운전하지 마세요. Since you drank a lot, don’t drive!
아직 학교에 있으니까 조금만 기다려 주세요. I am still at school, please wait for me a little bit more.
우리도 아침을 안 먹었으니까 같이 점심을 하자. We haven’t had breakfast yet, let’s go eat.
오늘은 피곤하니까 내일 가자. Because I’m tired today, let’s go tomorrow?