This is the 1891st most frequent Korean word.
그러니
"그러니" means "so" or "therefore."
Here, '그러니' is derived from '그러하니', meaning 'so' or 'therefore', to connect the situation of being late with the advice to rest.
시간이 늦었으니 쉬도록 하세요.
It is late, so you should rest.
In this case, '그러니' functions to indicate a reason (everyone knows) leading to a request (being quiet).
모두 알고 있으니 조용히 해 주세요.
Everyone knows, so please be quiet.
Here, '그러니' signifies cause and effect, linking the prior knowledge of answers with the expected success in the test.