This pattern shows one's desire to take advantage of some opportunity before one.

- in certain contexts, also shows an impetus or spontaneous influence of a certain fact or occurence
- used with the past modifier suffixes -ㄴ/은 and present modifier suffix -는
— is not used with the future modifier suffix -ㄹ/을
- used without the modifier suffix can be used with certain nouns to mean "due to", "on account of"

Examples:
부산에 온 김에 구경해야지요/Since you can to Pusan, you should go sightseeing.
우체국에 가는 김에, 이것은 좀 부쳐 주세요./As long as you are going to the post office, please mail this for me.
술 김에 싸웠어요./Due to alcohol (due to the influence of), I fought.

see also 바에야