만 먹으면

if you only eat

→ 만
phrase Uncommon #6,945
if you only eat 만 means “only” and attaches to the thing being eaten; 먹으면 means “if [someone] eats.”
KO: 이것만 먹으면 배고플 거예요.
EN: If you only eat this, you'll be hungry.

항상 고양이만 먹으면 분명히 질릴 거 야

If you only eat cats all the time, you'll definitely get tired of them.

— 순응자 (1971)

그렇게 콩만 먹으면 어떡해

You can’t just eat beans like that.

— 동경 이야기 (1953)

나이만 먹으면 돼요

You just need to get older.

— 레옹 (1994)