ㄴ가

"is it…? I wonder"

Turns a sentence into a question the speaker is mostly asking themselves: 두루미인가? is "is that a crane?", 이유가 돈인가? is "is money the reason?", 반전이 일어날 것인가? is "will there be a twist?". It has a musing, uncertain feel rather than a demand for an answer, which is why narrators and hosts use it to hang a question in the air. Adding 요 makes it a real, polite question to someone: 여는 건가요? is "are they opening one?". After a consonant-ending stem it appears as -은가.

Register: musing; 요 makes it polite