N + 이/가
Appending 이/가 to a noun denotes the subject of the sentence.
When the noun ends in a consonant, the ending 이 is used.
When the noun ends in a vowel, 가 is used.
The subject particle is used to indicate the subject of the sentence. This is a more general usage than the topic particle (which is used more specifically), and is often omitted when talking when the subject is obvious. That is why in Korean, you don’t need to say ‘I/me’ or re-iterate the subject all the time, unless its not obvious.
뭐가 좋아요? = What is good?
오늘은 날씨가 좋네요. = I’m working tomorrow.
시간이 없어요. = I don’t have any time.