가

marks what something becomes or isn't

Tags a vowel-ending noun as the thing something becomes or fails to be: 돼지가 돼가지고 is "I turn into a pig", 호랑이가 아니라 표범이다 is "it's not a tiger, it's a leopard", 메시가 될지 is "whether he'll become Messi". It has no English word of its own — it just points 되다 ("become") or 아니다 ("is not") at the right noun. Consonant-ending nouns take 이 in the same slot. It looks exactly like the subject marker 가, but here the noun is not the one acting; it is the identity being taken on or denied.